libstagefright: Don't fail when we don't get height/width from decoder

This fixes video for zeppelin, which was failing due to this
F/QComHardwareRenderer(  122): hardware/msm7k/libstagefrighthw/QComHardwareRenderer.cpp:78 mDecodedHeight > 0

Many thanks to Ricardo Cerqueira (arcee) for fixing this one

Change-Id: I17dac8bfbe7af0f319a00e3a4a70563b59749811