gst/gstutils.c: Add assertion that makes sure src_val is >=0, just like gst_query_new_convert() has. (#315895)

Original commit message from CVS:
Reviewed by: Tim-Philipp Müller  <tim at centricular dot net>
* gst/gstutils.c: (gst_pad_query_convert):
Add assertion that makes sure src_val is >=0, just like
gst_query_new_convert() has. (#315895)