Fix crash when there's unexpected non-YUV buffer input.

The lockYCbCr returns NO_ERROR when called on non-YUV buffer,
but all pointers in android_ycbcr are set to null.

Check that the buffer has expected pixel format and fail early
(and gracefully) when it doesn't.

Bug: 301023410
Test: atest VirtualCameraTest (modified)
Change-Id: I5af39c3d844392eac02f32ee55a50a2a2899466b