Fix up a bunch of places where a pointer into the packet buffer was cast

Fix up a bunch of places where a pointer into the packet buffer was cast
to a type requiring 2-byte or better alignment and was then
dereferenced; doing that requires that the code generated by your
compiler not trap if it makes an unaligned reference, and on most RISC
processors the code generated by the compiler *will* trap on an
unaligned reference by default.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@1480 f5534014-38df-0310-8fa8-9805f1628bb7