fix: Adds a null check in mod_limits_exceptions.

It is failing on prosody 0.11.4 with mod_limits_exception.lua:29: attempt to call method 'set_stanza_size_limit' (a nil value). That prosody misses set_stanza_size_limit.

resources/prosody-plugins/mod_limits_exception.lua

 			session.throttle = nil;
 		end
 
-		if unlimited_stanza_size_limit then
+		if unlimited_stanza_size_limit and session.stream.set_stanza_size_limit then
 			module:log('info', 'Setting stanza size limits for %s to %s', jid, unlimited_stanza_size_limit)
 			session.stream:set_stanza_size_limit(unlimited_stanza_size_limit);
 		end