Safely copy MIMEType to prevent use after free (Issue #14734) (#15313)

Author: JesusRojass

FirebasePerformance/Sources/Instrumentation/FPRNetworkTrace.m

@@ -251,7 +251,9 @@ - (void)didCompleteRequestWithResponse:(NSURLResponse *)response error:(NSError
       self.responseCode = (int32_t)HTTPResponse.statusCode;
     }
     self.responseError = error;
-    self.responseContentType = response.MIMEType;
+    // Safely copy MIMEType to prevent use after free
+    NSString *mime = [response.MIMEType copy];
+    self.responseContentType = (mime.length ? mime : nil);
     [self checkpointState:FPRNetworkTraceCheckpointStateResponseCompleted];
 
     // Send the network trace for logging.