Avoid crash when resizing window

Sanity check input to gui_macvim_draw_string(); it can happen that 'len'
is non-zero even though first byte of 's' is NUL (test case with file
containing Chinese characters).

src/MacVim/gui_macvim.m

@@ -323,8 +323,10 @@
     // wide and vice versa.
     for (i = 0; i < len; i += cl) {
         c = utf_ptr2char(s + i);
-        cl = utf_ptr2len(s + i);
         cn = utf_char2cells(c);
+        cl = utf_ptr2len(s + i);
+        if (0 == cl)
+            len = i;    // len must be wrong (shouldn't happen)
 
         if (!utf_iscomposing(c)) {
             if ((cn > 1 && !wide) || (cn <= 1 && wide)) {