Fix typo in pvq.c

This fixes an issue that was introduced in commit
f1a9307c8752883e8692476de72a0e1f9665ba30, "Add a 32x32 transform (and
inverse)."

Because of this bug, changing OD_DISABLE_QM to 1 in internal.h doesn't work.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>