mpeg4videoparse: fix criticals trying to insert configs that don't exist yet

With mpeg4videoparse drop=false config-interval=N|-1 we might be
trying to insert a config before we have actually received one,
in which case we'll try to map a NULL buffer which will generate
lots of criticals.

Fixes #855

Part-of: <!2558>

gst/videoparsers/gstmpeg4videoparse.c

@@ -793,7 +793,8 @@ gst_mpeg4vparse_pre_push_frame (GstBaseParse * parse, GstBaseParseFrame * frame)
 
         /* we need to send config now first */
         GST_INFO_OBJECT (parse, "inserting config in stream");
-        gst_buffer_map (mp4vparse->config, &cmap, GST_MAP_READ);
+        if (mp4vparse->config != NULL
+            && gst_buffer_map (mp4vparse->config, &cmap, GST_MAP_READ)) {
           diffconf = (gst_buffer_get_size (buffer) < cmap.size)
               || gst_buffer_memcmp (buffer, 0, cmap.data, cmap.size);
           csize = cmap.size;
@@ -814,6 +815,9 @@ gst_mpeg4vparse_pre_push_frame (GstBaseParse * parse, GstBaseParseFrame * frame)
           } else {
             GST_INFO_OBJECT (parse, "... but avoiding duplication");
           }
+        } else {
+          GST_WARNING_OBJECT (parse, "No config received yet");
+        }
 
         if (G_UNLIKELY (timestamp != -1)) {
           mp4vparse->last_report = timestamp;