cmdutils: Use the correct guard

The OptionDef arrays are terminated with a { NULL } element not NULL.

CC: libav-stable@libav.org
Bug-Id: CID 703769

cmdutils.c

@@ -414,7 +414,7 @@ int locate_option(int argc, char **argv, const OptionDef *options,
              (po->name && !strcmp(optname, po->name)))
             return i;
 
-        if (!po || po->flags & HAS_ARG)
+        if (!po->name || po->flags & HAS_ARG)
             i++;
     }
     return 0;