stagefright: avoid crash when histogram option is used

if decodeTimesUs vector is null then it will lead to a segfault when decodeTimesUs->itemAt is called

Test: stagefright -x -S -s <a_video_file>; stagefright -x -S -r <a_video_file>; stagefright -x -s <a_video_file>

Change-Id: I3a5f283d3eecfefa3bdb9ad84385697ab3b1738e
1 file changed