Fix SIGSEGV in callback after block is nil'd (#3786)

Author: paulb777

Firebase/Storage/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 3.4.1
+- [fixed] Fix crash in FIRStorageUploadTask (#3750).
+
 # 3.4.0
 - [fixed] Ensure that users don't accidently invoke `Storage()` instead of `Storage.storage()`.
   If your code calls the constructor of Storage directly, we will throw an assertion failure,

Firebase/Storage/FIRStorageUploadTask.m

@@ -141,8 +141,6 @@ - (void)enqueue {
     // Process fetches
     strongSelf.state = FIRStorageTaskStateRunning;
 
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Warc-retain-cycles"
     strongSelf->_fetcherCompletion = ^(NSData *_Nullable data, NSError *_Nullable error) {
       // Fire last progress updates
       [self fireHandlersForStatus:FIRStorageTaskStatusProgress snapshot:self.snapshot];
@@ -172,11 +170,12 @@ - (void)enqueue {
 
       [self finishTaskWithStatus:FIRStorageTaskStatusSuccess snapshot:self.snapshot];
     };
-#pragma clang diagnostic pop
 
     [strongSelf->_uploadFetcher
         beginFetchWithCompletionHandler:^(NSData *_Nullable data, NSError *_Nullable error) {
-          weakSelf.fetcherCompletion(data, error);
+          if (weakSelf.fetcherCompletion != nil) {
+            weakSelf.fetcherCompletion(data, error);
+          }
         }];
   }];
 }