Fix: MHP AMVP derivation

In some cases, the function getMultiHypMVPCands() use pu.addHypData[pu.numMergedAddHyps] instead of the input mhData for the derivation of AMVP list. Therefore possibly derive the AMVP list by a wrong given reference picture.
This would trigger software crash in non-CTC GOP structure, wherein the number of reference pictures for additional hypothesis is less than the number of reference pictures in List 0 or List 1. The fix has minor impact on CTC test result though (available class C and D results are -0.01% and 0.04%).