jabber: Only attempt to calculate hash for supported hashes

The way I initially ported jabber_caps_client_iqcb() to GChecksum
used G_CHECKSUM_SHA1 as a default hash type. This was to prevent
hash_type from being used uninitialized. However, it was pointed
out that it would probably be better to specifically only hash
when the hash type has been recognized. This patch refactors it
to do so.