ffmpeg: properly cleanup filter graph on init failure

The filter field is often used to check whether a filter is
configured. If configuring the filter actually fails somewhere in
the middle of it, these fields could still be set to non-NULL, which
lead to other code accessing the half-configured filter graph, which
in turn could lead to crashes within libavfilter.

Solve this by properly resetting all fields.

This was triggered by a fuzzed sample after the recent changes. It's
unknown whether this behavior could be triggered before that.
