Fix mutating array during iteration crash (#11669)

Author: danielamitay

FirebaseABTesting/Sources/FIRExperimentController.m

@@ -92,7 +92,7 @@
   // Check if the experiment is in experiments but not payloads.
   for (id experiment in [experiments copy]) {
     BOOL doesExperimentNoLongerExist = YES;
-    for (NSData *payload in payloads) {
+    for (NSData *payload in [payloads copy]) {
       ABTExperimentPayload *experimentPayload = ABTDeserializeExperimentPayload(payload);
       if (!experimentPayload) {
         FIRLogInfo(kFIRLoggerABTesting, @"I-ABT000002",