Fix #1596 on encoding crash when DeltaQpRD is used
It was reported that the issue disappeared when REUSE_CU_RESULTS is off. REUSE_CU_RESULTS checks whether the cached data is valid with function isValid(), in which whether CU QP is different from cached QP is also checked. However, the cached data is not properly reset when DeltaQpRD is used. This fix is to reset QP value to let isValid() correctly detect such cases.