Fix: prevent an empty merge candidate list for amvpMerge for non-CTC
merge candidate list could be empty due to the limitation of considering reference index to min(L0, L1) on the non-CTC configuration.
for example,
- when POC of the current picture is N, reference pictures is constructed to have poc{N+32, N+16, N-16} for L0, poc{N+32} for L1 in reference picture list.
- amvpMergeModeFlag[0] is true.
only the zero motion vector with reference index 0 is considered. however reference index 0 indicates the reference picture which is poc N+32. this reference index is not valid pair of amvpMerge. therefore the merge candidate list is made an empty list.