SoftMPEG4: Check the buffer size before writing the reference frame.

Also prevent overflow in SoftMPEG4 and division by zero in SoftMPEG4Encoder.

Bug: 30033990
Change-Id: I7701f5fc54c2670587d122330e5dc851f64ed3c2
(cherry picked from commit 695123195034402ca76169b195069c28c30342d3)
