HALFQP should only be added to the inverse quantizer when the block is
coded with PQUANT. When PQUANT is equal to ALTPQUANT, the original test
for the addition of HALFQP fails. A negative value for mquant indicates
that the value was derived from VOPDQUANT.

Fixes #4372
Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>