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 #5095)

Author: brammool

src/ex_cmds.c

@@ -251,18 +251,23 @@ 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 = ml_get_curline();
+    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 && VIM_ISWHITE(last[-1]); --last)
 	;
     save = *last;
     *last = NUL;
-    len = linetabsize(line);		/* get line length */
-    if (has_tab != NULL)		/* check for embedded TAB */
+    len = linetabsize(line);		// get line length
+    if (has_tab != NULL)		// check for embedded TAB
 	*has_tab = (vim_strchr(first, TAB) != NULL);
     *last = save;
 

src/testdir/test_textformat.vim

@@ -1,4 +1,7 @@
 " Tests for the various 'formatoptions' settings
+
+source check.vim
+
 func Test_text_format()
   enew!
 
@@ -489,3 +492,20 @@ func Test_format_list_auto()
   bwipe!
   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

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2198,
 /**/
     2197,
 /**/