Update ffmpeg.c to work with newer versions of FFmpeg

Since ffmpeg 2.0, the macro AVCODEC_MAX_AUDIO_FRAME_SIZE is deprecated
and removed. This commit simply defines the macro if it's not defined
previously, using the same value it used to be in old versions of
libavcodec.

Actually, as my branch has been using swr_convert, where caller specifies
the size of output buffer, even if the current size of 192000 proved to
be not enough in the future, we can simply let swr_convert deal with the
insufficient size by a small modification.

Currently the parameters to swr_convert() still assumes that the size of
192000 samples is always enough for the output buffer.

Signed-off-by: Gregory Petrosyan <gregory.petrosyan@gmail.com>