patch 8.1.2325: crash when using balloon with empty line

Problem:    Crash when using balloon with empty line.
Solution:   Handle empty lines. (Markus Braun)

src/popupmenu.c

@@ -1209,12 +1209,16 @@ split_message(char_u *mesg, pumitem_T **array)
 	int	cells;
 
 	item = ((balpart_T *)ga.ga_data) + item_idx;
+	if (item->bytelen == 0)
+	    (*array)[line++].pum_text = vim_strsave((char_u *)"");
+	else
 	    for (skip = 0; skip < item->bytelen; skip += thislen)
 	    {
 		if (split_long_items && item->cells >= BALLOON_MIN_WIDTH)
 		{
 		    cells = item->indent * 2;
-		for (p = item->start + skip; p < item->start + item->bytelen;
+		    for (p = item->start + skip;
+			    p < item->start + item->bytelen;
 							    p += mb_ptr2len(p))
 			if ((cells += ptr2cells(p)) > BALLOON_MIN_WIDTH)
 			    break;

src/testdir/test_popup.vim

@@ -795,6 +795,12 @@ func Test_balloon_split()
         \ '  next = 123}',
         \ ], balloon_split(
         \ 'struct = 0x234 {long = 2343 "\\"some long string that will be wrapped in two\\"", next = 123}'))
+  call assert_equal([
+        \ 'Some comment',
+        \ '',
+        \ 'typedef this that;',
+        \ ], balloon_split(
+        \ "Some comment\n\ntypedef this that;"))
 endfunc
 
 func Test_popup_position()

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2325,
 /**/
     2324,
 /**/