basesrc: fix pool leak on allocation query error path

It could be triggered by:

gst-launch-1.0 videotestsrc num-buffers=20 ! videcrop bottom=214748364 ! videoconvert ! autovideosink

Spotted while testing:

https://bugzilla.gnome.org/show_bug.cgi?id=743910

libs/gst/base/gstbasesrc.c

@@ -3112,6 +3112,7 @@ gst_base_src_decide_allocation_default (GstBaseSrc * basesrc, GstQuery * query)
   return TRUE;
 
 config_failed:
+  gst_object_unref (pool);
   GST_ELEMENT_ERROR (basesrc, RESOURCE, SETTINGS,
       ("Failed to configure the buffer pool"),
       ("Configuration is most likely invalid, please report this issue."));