fftools: do not access out of bounds filtergraph

fftools: do not access out of bounds filtergraph

The log message was logged for `filtergraphs[j]` which would cause a
heap buffer overflow in certain circumstances.

Correctly it should be logged for the current filtergraph, so just
use `fg` here.

(cherry picked from commit 5beeb3a1f97d8f6d4076fe83aaf5e2e5871f945e)

Gbp-Pq: Name 0005-fftools-do-not-access-out-of-bounds-filtergraph.patch