MPEG4 P10 packetizer: avoid integer overflow

The get_uintXZ() functions all return uint32_t. If garbage data causes
those functions to return max(uint32_t) - m_nalu_size_len_dst (mostly
4) then nalu_size will become 0, and the check for »beyond end of
buffer« will fail resulting in an endless loop.

Happens e.g. for http://ftp.ccc.de/congress/2013/mp4-hd/30c3-5423-en-The_Tor_Network_h264-hd.mp4
cosmetics: alignment