correct quantizer initialization

The encoder was not correctly catching transitions in the quantizer
deltas. If a delta_q was set, then the quantizer would be reinitialized
on every frame, but if they transitioned to 0, the quantizer would
not be reinitialized, leading to a encode-decode mismatch.

This bug was triggered by commit 999e155f, which sets a Y2 delta Q
for very low base Q levels.

Change-Id: Ia6733464a55ee4ff2edbb82c0873980d345446f5