avfiltergraph: silence an uninitialized variable warning

The warning is:
libavfilter/avfiltergraph.c: In function ‘avfilter_graph_config’:
libavfilter/avfiltergraph.c:528:9: warning: ‘best_idx’ may be used uninitialized in this function [-Wuninitialized]
libavfilter/avfiltergraph.c:479:13: note: ‘best_idx’ was declared here

Initialize it to an invalid value and add an assert that it's properly
set later.