gst/parse/grammar.y: fix assert to not trigger when there's no error argument

gst/parse/grammar.y: fix assert to not trigger when there's no error argument

Original commit message from CVS:
* gst/parse/grammar.y:
fix assert to not trigger when there's no error argument
* gst/parse/parse.l:
fix definition of caps to allow more than two structures
* testsuite/caps/Makefile.am:
* testsuite/caps/renegotiate.c: (main):
it's sinesrc and works in that case