Fix incorrect register assert (#139)

Asserting !registered on unregister doesn't make sense, instead if we want to UNREGISTER we want to make sure we are REGISTERED

Author: VladimirBrejcha

lib/src/sip_ua_helper.dart

@@ -61,7 +61,7 @@ class SIPUAHelper extends EventManager {
 
  void unregister([bool all = true]) {
  if (_ua != null) {
- assert(!registered, 'ERROR: you must call register first.');
+ assert(registered, 'ERROR: you must call register first.');
  _ua.unregister(all: all);
  } else {
  Log.e('ERROR: unregister called, you must call start first.');