Audio frame sizes were already being checked for overflow,
but video frame sizes were taken as-is, which would
lead to heap corruption.

In case an overflow has been detected, playback is aborted immediately as
there is no way we can ever recover from such a situation due to the lack
of a (well-known) frame header signature that could be used to skip over
the corrupted part of the streams.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>