libiberty/md5: fix strict alias warnings

libiberty/md5: fix strict alias warnings

Current libiberty md5 code triggers these warnings with gcc-4.7.1 for me:

libiberty/md5.c: In function â\80\98md5_finish_ctxâ\80\99:
libiberty/md5.c:117:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
libiberty/md5.c:118:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

The change below fixes things for me.  The optimized output (-O2) is the same
before/after my change on x86_64-linux.  I imagine it'll be the same for most
targets.  It seems simpler than using a union on the md5_ctx buffer since these
are the only two locations in the code where this occurs.