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)
396b7c78c0

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

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.

src/nvim/testdir/check.vim

@@ -1,6 +1,30 @@
 source shared.vim
 source term_util.vim
 
+" Command to check for the presence of a feature.
+command -nargs=1 CheckFeature call CheckFeature(<f-args>)
+func CheckFeature(name)
+  if !has(a:name)
+    throw 'Skipped: ' .. a:name .. ' feature missing'
+  endif
+endfunc
+
+" Command to check for the presence of a working option.
+command -nargs=1 CheckOption call CheckOption(<f-args>)
+func CheckOption(name)
+  if !exists('+' .. a:name)
+    throw 'Skipped: ' .. a:name .. ' option not supported'
+  endif
+endfunc
+
+" Command to check for the presence of a function.
+command -nargs=1 CheckFunction call CheckFunction(<f-args>)
+func CheckFunction(name)
+  if !exists('*' .. a:name)
+    throw 'Skipped: ' .. a:name .. ' function missing'
+  endif
+endfunc
+
 " Command to check that making screendumps is supported.
 " Caller must source screendump.vim
 command CheckScreendump call CheckScreendump()

src/nvim/testdir/test_textformat.vim

@@ -1,4 +1,7 @@
 " Tests for the various 'formatoptions' settings
+
+source check.vim
+
 func Test_text_format()
   enew!
 
@@ -490,6 +493,23 @@ func Test_format_list_auto()
   set fo& ai& bs&
 endfunc
 
+func Test_crash_github_issue_5095()
+  CheckFeature autocmd
+
+  " This used to segfault, see https://github.com/vim/vim/issues/5095
+  augroup testing
+    au BufNew x center
+  augroup END
+
+  next! x
+
+  bw
+  augroup testing
+    au!
+  augroup END
+  augroup! testing
+endfunc
+
 " Test for formatting multi-byte text with 'fo=t'
 func Test_tw_2_fo_t()
   new