avfiltergraph: check for sws opts being non-NULL before using them.

Avoid snprintfing a NULL pointer.

CC: libav-stable@libav.org
Merge remote-tracking branch 'qatar/master'

* qatar/master:
  avfiltergraph: check for sws opts being non-NULL before using them.

Conflicts:
	libavfilter/avfiltergraph.c
	libavfilter/graphparser.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Fix make checkheaders without opencl headers.