libswresample: check input to swr_convert_frame for NULL

When 'out' is an AVFrame that does not have buffers preallocated,
swr_convert_frame tries to allocate buffers of the right size. However
in calculating this size it failed to check for whether 'in' is NULL
(requesting that swr's internal buffers are to be flushed).

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>