media_profiles: the HFR parameters don't need to be mandatory

Change-Id: I5e2d32f3be31f2ef82c314088d032105e6c8a5f8
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