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Avoid divide-by-zero in vp8 initialization

Check that the numerator is not zero. If it is, guess 30fps.

Fixes a clang IOC error in the quantize test. It's very unlikely for
this to occur in the wild because the setup in the quantize test is very
nonstandard.

Change-Id: Icdab7b81d4e168d3423e14db20787f960052e0c3

--- a/vp8/encoder/onyx_if.c

+++ b/vp8/encoder/onyx_if.c

@@ -1363,15 +1363,20 @@

     cm->version = oxcf->Version;

     vp8_setup_version(cm);

-    /* frame rate is not available on the first frame, as it's derived from

+    /* Frame rate is not available on the first frame, as it's derived from

      * the observed timestamps. The actual value used here doesn't matter

-     * too much, as it will adapt quickly. If the reciprocal of the timebase

-     * seems like a reasonable framerate, then use that as a guess, otherwise

-     * use 30.

+     * too much, as it will adapt quickly.

      */

-    cpi->framerate = (double)(oxcf->timebase.den) /

-                     (double)(oxcf->timebase.num);

+    if (oxcf->timebase.num > 0) {

+      cpi->framerate = (double)(oxcf->timebase.den) /

+                       (double)(oxcf->timebase.num);

+    } else {

+      cpi->framerate = 30;

+    }

+    /* If the reciprocal of the timebase seems like a reasonable framerate,

+     * then use that as a guess, otherwise use 30.

+     */

     if (cpi->framerate > 180)

         cpi->framerate = 30;