patch 9.0.1995: Invalid memory access with empty 'foldexpr'

Problem:  Invalid memory access when 'foldexpr' returns empty string.
Solution: Check for NUL.

closes: #13293

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/eval.c

@@ -968,7 +968,7 @@ eval_foldexpr(win_T *wp, int *cp)
 	    // If the result is a string, check if there is a non-digit before
 	    // the number.
 	    s = tv.vval.v_string;
-	    if (!VIM_ISDIGIT(*s) && *s != '-')
+	    if (*s != NUL && !VIM_ISDIGIT(*s) && *s != '-')
 		*cp = *s++;
 	    retval = atol((char *)s);
 	}

src/testdir/test_fold.vim

@@ -1769,4 +1769,13 @@ func Test_foldcolumn_linebreak_control_char()
 
   bwipe!
 endfunc
+
+" This used to cause invalid memory access
+func Test_foldexpr_return_empty_string()
+  new
+  setlocal foldexpr='' foldmethod=expr
+  redraw
+
+  bwipe!
+endfunc
 

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1995,
 /**/
     1994,
 /**/