Fix: Initialization of m_amvpMergeModeValidRefIdx
One of the list that are used for checking if a reference index is valid was only initialized according to the length of list 0 when it shall be according to the length of list 1.
One of the list that are used for checking if a reference index is valid was only initialized according to the length of list 0 when it shall be according to the length of list 1.