Throw in some casts to squelch warnings, at least with some compilers

Throw in some casts to squelch warnings, at least with some compilers
(the values are const because nobody's supposed to modify them once
they've been allocated, but they *can* be freed - by us - when we're
done with them).

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@14936 f5534014-38df-0310-8fa8-9805f1628bb7