Fix crash where uninitialized password is freed.

Inside imap_mailstorage_init_sasl_with_local_address,
imap_storage->imap_password was only initialized if the password
parameter was set.  This meant that we would crash in
imap_mailstorage_uninitialize when attempting to free the
uninitialized value.
Merge pull request #87 from tipbit/uninitialized-password

Fix crash where uninitialized password is freed.