Make the object attached to a frame for VJ decompression be a structure

Make the object attached to a frame for VJ decompression be a structure
containing a 4-byte offset and 128 bytes of data, rather than a 1-byte
offset and 128 bits of data, so that the data is aligned on a 4-byte
boundary and doesn't upset processors with strict alignment
requirements.

We don't free individual memory chunks containing those objects, so make
the memory chunk pool an "allocate-only" pool.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4428 f5534014-38df-0310-8fa8-9805f1628bb7