Fix crash in RCTLegacyViewManagerInteropCoordinator

Summary:
Like the task mentions `strongSelf->_eventInterceptors` was crashing, probably because the coordinator was cleaned up before this block ran.

Check to make sure self is still valid before attempting to access any instance variables.

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D25073812

fbshipit-source-id: cdf666f2ac028b5523097f15ff51fbae9f9ffbd8

ReactCommon/react/renderer/components/legacyviewmanagerinterop/RCTLegacyViewManagerInteropCoordinator.mm

@@ -40,9 +40,11 @@ - (instancetype)initWithComponentData:(RCTComponentData *)componentData bridge:(
     __weak __typeof(self) weakSelf = self;
     _componentData.eventInterceptor = ^(NSString *eventName, NSDictionary *event, NSNumber *reactTag) {
       __typeof(self) strongSelf = weakSelf;
-      InterceptorBlock block = [strongSelf->_eventInterceptors objectForKey:reactTag];
-      if (block) {
-        block(std::string([RCTNormalizeInputEventName(eventName) UTF8String]), convertIdToFollyDynamic(event ?: @{}));
+      if (strongSelf) {
+        InterceptorBlock block = [strongSelf->_eventInterceptors objectForKey:reactTag];
+        if (block) {
+          block(std::string([RCTNormalizeInputEventName(eventName) UTF8String]), convertIdToFollyDynamic(event ?: @{}));
+        }
       }
     };
   }