terminal: Avoid invalid cursor col (#6265)

Patch-by: oni-link <knil.ino@gmail.com>

Closes #6203

https://s3.amazonaws.com/archive.travis-ci.org/jobs/206794197/log.txt

References #3161

    [ RUN      ] ...d/neovim/neovim/test/functional/terminal/buffer_spec.lua @ 199: terminal buffer term_close() use-after-free #4393
    ./test/functional/helpers.lua:187: attempt to perform arithmetic on local 'written' (a nil value)

    stack traceback:
    	./test/functional/helpers.lua:187: in function 'nvim_feed'
    	./test/functional/helpers.lua:329: in function 'execute'
    	...d/neovim/neovim/test/functional/terminal/buffer_spec.lua:206: in function <...d/neovim/neovim/test/functional/terminal/buffer_spec.lua:199>

    [  ERROR   ] ...d/neovim/neovim/test/functional/terminal/buffer_spec.lua @ 199: terminal buffer term_close() use-after-free #4393 (199.47 ms)
    ==================== File /home/travis/build/neovim/neovim/build/log/ubsan.15466 ====================
    = =================================================================
    = ==15466==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x621000029101 at pc 0x000000ea7ba0 bp 0x7ffd5bb628c0 sp 0x7ffd5bb628b8
    = READ of size 1 at 0x621000029101 thread T0
    =     #0 0xea7b9f in utf_head_off /home/travis/build/neovim/neovim/src/nvim/mbyte.c:1637:7
    =     #1 0xeaaf53 in mb_adjustpos /home/travis/build/neovim/neovim/src/nvim/mbyte.c:1840:16
    =     #2 0xeaab48 in mb_adjust_cursor /home/travis/build/neovim/neovim/src/nvim/mbyte.c:1825:3
    =     #3 0x11000d0 in normal_finish_command /home/travis/build/neovim/neovim/src/nvim/normal.c:928:5
    =     #4 0x1077df1 in normal_execute /home/travis/build/neovim/neovim/src/nvim/normal.c:1147:3
    =     #5 0x16ff943 in state_enter /home/travis/build/neovim/neovim/src/nvim/state.c:58:26
    =     #6 0x102d8db in normal_enter /home/travis/build/neovim/neovim/src/nvim/normal.c:463:3
    =     #7 0xdf3398 in main /home/travis/build/neovim/neovim/src/nvim/main.c:540:3
    =     #8 0x2b973e8b4f44 in __libc_start_main /build/eglibc-oGUzwX/eglibc-2.19/csu/libc-start.c:287
    =     #9 0x447445 in _start (/home/travis/build/neovim/neovim/build/bin/nvim+0x447445)
    =
    = 0x621000029101 is located 1 bytes to the right of 4096-byte region [0x621000028100,0x621000029100)
    = allocated by thread T0 here:
    =     #0 0x4f17b8 in malloc (/home/travis/build/neovim/neovim/build/bin/nvim+0x4f17b8)
    =     #1 0xf1f374 in try_malloc /home/travis/build/neovim/neovim/src/nvim/memory.c:84:15
    =     #2 0xf1f534 in xmalloc /home/travis/build/neovim/neovim/src/nvim/memory.c:118:15
    =     #3 0xebe6a8 in mf_alloc_bhdr /home/travis/build/neovim/neovim/src/nvim/memfile.c:646:17
    =     #4 0xebc394 in mf_new /home/travis/build/neovim/neovim/src/nvim/memfile.c:297:12
    =     #5 0xed1368 in ml_new_data /home/travis/build/neovim/neovim/src/nvim/memline.c:2704:16
    =     #6 0xece6ab in ml_open /home/travis/build/neovim/neovim/src/nvim/memline.c:349:8
    =     #7 0x6438ad in open_buffer /home/travis/build/neovim/neovim/src/nvim/buffer.c:109:7
    =     #8 0xa6ec8d in do_ecmd /home/travis/build/neovim/neovim/src/nvim/ex_cmds.c:2489:24
    =     #9 0xb5a0f9 in do_exedit /home/travis/build/neovim/neovim/src/nvim/ex_docmd.c:6723:9
    =     #10 0xb791f8 in ex_edit /home/travis/build/neovim/neovim/src/nvim/ex_docmd.c:6651:3
    =     #11 0xb28b43 in do_one_cmd /home/travis/build/neovim/neovim/src/nvim/ex_docmd.c:2198:5
    =     #12 0xb077a7 in do_cmdline /home/travis/build/neovim/neovim/src/nvim/ex_docmd.c:601:20
    =     #13 0x10905db in nv_colon /home/travis/build/neovim/neovim/src/nvim/normal.c:4495:18
    =     #14 0x1077de8 in normal_execute /home/travis/build/neovim/neovim/src/nvim/normal.c:1144:3
    =     #15 0x16ff943 in state_enter /home/travis/build/neovim/neovim/src/nvim/state.c:58:26
    =     #16 0x102d8db in normal_enter /home/travis/build/neovim/neovim/src/nvim/normal.c:463:3
    =     #17 0xdf3398 in main /home/travis/build/neovim/neovim/src/nvim/main.c:540:3
    =     #18 0x2b973e8b4f44 in __libc_start_main /build/eglibc-oGUzwX/eglibc-2.19/csu/libc-start.c:287
    =
    = SUMMARY: AddressSanitizer: heap-buffer-overflow /home/travis/build/neovim/neovim/src/nvim/mbyte.c:1637:7 in utf_head_off
    stack traceback:
    	./test/helpers.lua:80: in function 'check_logs'
    	./test/functional/helpers.lua:639: in function <./test/functional/helpers.lua:638>

    [----------] 9 tests from /home/travis/build/neovim/neovim/test/functional/terminal/buffer_spec.lua (2263.12 ms total)

0 files changed, 0 insertions, 0 deletions