fix: Future already completed

[tejainece]

Happens on Mac after recovering from suspend.

[ERROR:flutter/runtime/dart_vm_initializer.cc(40)] Unhandled Exception: Bad state: Future already completed
#0      _AsyncCompleter.complete (dart:async/future_impl.dart:84:31)
#1      SoLoud._initializeNativeCallbacks.<anonymous closure> (package:flutter_soloud/src/soloud.dart:423:18)
#2      _RootZone.runUnaryGuarded (dart:async/zone.dart:1609:10)
#3      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:366:11)
#4      _DelayedData.perform (dart:async/stream_impl.dart:542:14)
#5      _PendingEvents.handleNext (dart:async/stream_impl.dart:647:11)
#6      _PendingEvents.schedule.<anonymous closure> (dart:async/stream_impl.dart:618:7)
#7      _microtaskLoop (dart:async/schedule_microtask.dart:40:21)
#8      _startMicrotaskLoop (dart:async/schedule_microtask.dart:49:5)