lavc/qsvenc: replace assert with error return

lavc/qsvenc: replace assert with error return

Fix the (m)jpeg encoding regression issue as decription in tikect #7593,
due to bs->FrameType is not set in such case in
MSDK (https://github.com/Intel-Media-SDK/MediaSDK/issues/970).
(And assert on a value coming from an external library is not proper.)

Add default type check for bs->FrameType, and return invalid data error in function
ff_qsv_encode to avoid using uninitialized value.

Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Zhong Li <zhong.li@intel.com>