patch 8.0.0561: undefined behavior when using backslash after empty line

Problem:    Undefined behavior when using backslash after empty line.
Solution:   Check for an empty line. (Dominique Pelle, closes #1631)

src/misc2.c

@@ -2129,7 +2129,7 @@ ga_concat(garray_T *gap, char_u *s)
 {
     int    len;
 
-    if (s == NULL)
+    if (s == NULL || *s == NUL)
 	return;
     len = (int)STRLEN(s);
     if (ga_grow(gap, len) == OK)

src/testdir/test_vimscript.vim

@@ -1309,6 +1309,17 @@ func Test_script_lines()
     endtry
 endfunc
 
+"-------------------------------------------------------------------------------
+" Test 96:  line continuation						    {{{1
+"
+"           Undefined behavior was detected by ubsan with line continuation
+"           after an empty line.
+"-------------------------------------------------------------------------------
+func Test_script_emty_line_continuation()
+
+    \
+endfunc
+
 "-------------------------------------------------------------------------------
 " Modelines								    {{{1
 " vim: ts=8 sw=4 tw=80 fdm=marker

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    561,
 /**/
     560,
 /**/