avformat/flvenc: Forward errors from allocating keyframe index

avformat/flvenc: Forward errors from allocating keyframe index

While the function adding a new element to the keyframe index checked
the allocation, the caller didn't check the return value. This has been
changed. To do so, the return value has been changed to an ordinary ret
instead of pb->error. This doesn't pose a problem, as write_packet() in
mux.c already checks for write errors (since 9ad1e0c1).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>