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Problem:    illegal memory access when completing with invalid bytes.
Solution:   Avoid going over the end of the completion text.

vim/vim@4b28ba3

Co-authored-by: Bram Moolenaar <Bram@vim.org>

fix: set full_screen when in ex_mode

Problem Description: In ex_mode, the default_grid.chars are not allocated, and subsequently,
the w_grid.target in curwin is not allocated to default_grid in update_screen. This leads to
a null pointer crash when the completion function is executed in ex_mode.

Solution: Set full_screen when in ex_mode to ensure that default_grid is allocated.

`_FORTIFY_SOURCE` on Ubuntu caught this, resulting in:

    [OLDTEST] Running test_rename
    Failed: test_rename :: Nvim exited with non-zero code
    Job exited with code 134
    Screen (23 lines)
    ================================================================================
    "test_rename.vim" "test_rename.vim" 120L, 3623B
    Executing Test_rename_copy()
    Executing Test_rename_dir_to_dir()
    Executing Test_rename_fails()
    Error detected while processing command line..script /<<BUILDDIR>>/neovim-0.9.5/test/old/testdir/runtest.vim[437]..function RunTheTest[44]..Test_rename_fails:
    line   17:
    E730: using List as a String
    line   18:
    E976: using Blob as a String
    Executing Test_rename_file_ignore_case()*** buffer overflow detected ***: terminated

`snprintf`'s second parameter should be no greater than the number of
remaining bytes in the allocated object.  We can see that this was off
by one, because in the simple case where `tail == tempname` (for a file
in the current directory), `rename_with_tmp` was passing `MAXPATHL + 2`
for an object allocated with a size of only `MAXPATHL + 1`.

Introduced in 5f1a153831d480180b2203120cff776d771ec1a4.