libstagefright: Add NULL check for mHFRRatio

Commit 12fa017cb "libstagefright: Enable HFR feature for video recording"
is not yet fully implemented. Camera recording will fail because timeScale
is not calculated properly. Add a simple NULL check to fix.

Change-Id: Ieea78efb43645f57615309a91508092433bc34b3
Merge remote-tracking branch 'cm/cm-11.0' into HEAD