Fix divide by 0 for calculating vsyncs in buffer stuffing recovery

Arithmetic error when calculating number of vsyncs that have occurred
since last callback. Default number of vsyncs passed should be 0
whenever mLastFrameIntervalNanos is an invalid divisor.

Change-Id: I2e357b1cad9cd98c6bd7143100a7892c64a0bcbe
Fixes: 378981155
Test: presubmit
Flag: EXEMPT bug fix
1 file changed