fix a segfault if -x264encopts is the last commandline option (with no argument)

git-svn-id: svn://svn.mplayerhq.hu/mplayer/trunk@22050 b3059339-0415-0410-9bf9-f77b7e298cf2

libmpcodecs/ve_x264.c

@@ -86,6 +86,11 @@ void x264enc_set_param(m_option_t* opt, char* arg)
         initted = 1;
     }
 
+    if(!arg) {
+        parse_error = 1;
+        return;
+    }
+
     while(*arg) {
         char *name = arg;
         char *value;