avcodec/encode: Zero padding in ff_get_encode_buffer()

avcodec/encode: Zero padding in ff_get_encode_buffer()

The documentation of the get_encode_buffer() callback does not require
to zero the padding; therefore we do it in ff_get_encode_buffer().
This also constitutes an implicit check for whether the buffer is
actually allocated with padding.

The memset in avcodec_default_get_encode_buffer() is now redundant and
has been removed.

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>