vim-patch:8.1.2198: crash when using :center in autocommand

Problem:    Crash when using :center in autocommand.
Solution:   Bail out early for an empty line. (Dominique pelle, closes vim/vim#5095)
https://github.com/vim/vim/commit/396b7c78c0fd9cd07528963b18c27398491df40d

Cherry-pick 'src/testdir/check.vim' changes from patch 8.1.1544.

1 files changed, 9 insertions, 4 deletions

diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 519978f4fb..2bac6cba58 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -326,14 +326,19 @@ static int linelen(int *has_tab)
   int save;
   int len;
 
-  /* find the first non-blank character */
+  // Get the line.  If it's empty bail out early (could be the empty string
+  // for an unloaded buffer).
   line = get_cursor_line_ptr();
+  if (*line == NUL) {
+    return 0;
+  }
+  // find the first non-blank character
   first = skipwhite(line);
 
-  /* find the character after the last non-blank character */
+  // find the character after the last non-blank character
   for (last = first + STRLEN(first);
-       last > first && ascii_iswhite(last[-1]); --last)
-    ;
+       last > first && ascii_iswhite(last[-1]); last--) {
+  }
   save = *last;
   *last = NUL;
   // Get line length.