Respect strict aliasing in sha1 implementation

Changed wbuffer to union to silence this warning:

   hash_sha1.c: In function 'common64_end':
   hash_sha1.c:227:13: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Martin Milata <mmilata@redhat.com>