avfilter/bufferqueue: Increase buffer queue size

Avoid buffer overruns when processing some MOV files with the amerge
filter. Files produced by Adobe Premiere Pro CC have up to one second of audio
not interleaved. With common settings (<= 48kHz) that makes up to 47 frames so
a queue length of 64 makes sense.

Fixes ticket #3510.

Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>