Assert in function ff_qsv_encode to avoid using uninitialized value:

FF_DISABLE_DEPRECATION_WARNINGS
        avctx->coded_frame->pict_type = pict_type;
FF_ENABLE_DEPRECATION_WARNINGS
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Zhong Li <zhong.li@intel.com>