Fetch RTP option and extension data using EXTRACT_32BITS().

Fetch RTP option and extension data using EXTRACT_32BITS().

Do not assume that you're on a big-endian processor that can dereference
unaligned pointers - all the world's not a m68k.  :-) Most of the world
is little-endian these days by CPU count (especially on personal
computers), and one of the popular big-endian processor families, SPARC,
traps when dereferencing unaligned pointers.

While we're at it, note the last I-D that described RTPv1 and the
current RFC for RTPv2.