Philipp Zabel authored Mar 07, 2019 and Philipp Zabel committed Mar 08, 2019

This fixes a critical warning if the last-sample property is enabled:

  (gst-launch-1.0:391): GStreamer-CRITICAL **: 01:12:57.428: gst_object_unref: assertion 'object != NULL' failed

If the allocation query does not contain any allocation pools,
gst_query_parse_nth_allocation_pool will leave the local pool,
min, and max variables undefined, so check the array length first.
If pool is NULL, do not call gst_object_unref.