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 branch 'cm-11.0' of git://github.com/CyanogenMod/android_frameworks_av into cd-4.4