Check for start code emulation prevention

Start codes work because the three byte sequences 0x000000, 0x000001 and
0x000002 are illegal within a NALU. So when creating a NALU, care is
taken to escape these values that could otherwise be confused with a
start code. This is accomplished by inserting an ‘Emulation Prevention’
byte 0x03, so that 0x000001 becomes 0x00000301.