Fix crash in FIRInstanceID (#3465)

Author: morganchen12

Firebase/InstanceID/FIRIMessageCode.h

@@ -44,6 +44,8 @@ typedef NS_ENUM(NSInteger, FIRInstanceIDMessageCode) {
   kFIRInstanceIDMessageCodeInstanceID014 = 3014,
   kFIRInstanceIDMessageCodeInstanceID015 = 3015,
   kFIRInstanceIDMessageCodeRefetchingTokenForAPNS = 3016,
+  kFIRInstanceIDMessageCodeInstanceID017 = 3017,
+  kFIRInstanceIDMessageCodeInstanceID018 = 3018,
   // FIRInstanceIDAuthService.m
   kFIRInstanceIDMessageCodeAuthService000 = 5000,
   kFIRInstanceIDMessageCodeAuthService001 = 5001,

Firebase/InstanceID/FIRInstanceID.m

@@ -1012,6 +1012,16 @@ - (void)notifyAPNSTokenIsSet:(NSNotification *)notification {
     FIRInstanceID_WEAKIFY(self);
     [self asyncLoadKeyPairWithHandler:^(FIRInstanceIDKeyPair *keyPair, NSError *error) {
       FIRInstanceID_STRONGIFY(self);
+      if (self == nil) {
+        FIRInstanceIDLoggerError(kFIRInstanceIDMessageCodeInstanceID017,
+                                 @"Instance ID shut down during token reset. Aborting");
+        return;
+      }
+      if (self.apnsTokenData == nil) {
+        FIRInstanceIDLoggerError(kFIRInstanceIDMessageCodeInstanceID018,
+                                 @"apnsTokenData was set to nil during token reset. Aborting");
+        return;
+      }
 
       NSMutableDictionary *tokenOptions = [@{
         kFIRInstanceIDTokenOptionsAPNSKey : self.apnsTokenData,