Use avfilter_graph_alloc() instead of av_mallocz(sizeof(AVFilterGraph))
to allocate an AVFilterGraph; this also properly allocates the graph's
internal. The current code just happened to work because it did not
make any use of said internal.

Also check the allocation; this fixes Coverity #1292528.
Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>