omxvideodec: fix pool caps reference stealing

gst_buffer_pool_config_get_params() doesn't ref the returning caps;
so gst_caps_replace() was unreffing the reference owned by the pool.

https://bugzilla.gnome.org/show_bug.cgi?id=796918

omx/gstomxvideodec.c

@@ -721,7 +721,7 @@ gst_omx_video_dec_allocate_output_buffers (GstOMXVideoDec * self)
       max = min;
     } else if (max < min) {
       /* Can't use pool because can't have enough buffers */
-      gst_caps_replace (&caps, NULL);
+      caps = NULL;
     } else {
       min = max;
     }