frame->enc_ctrl.Payload is malloced in get_free_frame, directly memset
the whole structure of enc_ctrl to zero will cause the memory leak for
enc_ctrl.Payload.

frame->enc_ctrl as a structure will be malloc and init to zero by calling
frame = av_mallocz(sizeof(*frame)), so the memset is redundant and can
be removed.
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Zhong Li <zhong.li@intel.com>