should_heartbeat: Ensure prekey variable is always availabe in scope

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

slixmpp_omemo/__init__.py

@@ -593,6 +593,7 @@ class XEP_0384(BasePlugin):
             OMEMO_BASE_NS, self._device_id))
         # Don't error out. If it's not encrypted to us we don't need to send a
         # heartbeat.
+        prekey = False
         if key is not None:
             key = Key(key)
             prekey = key['prekey'] in TRUE_VALUES