Fix global.__fetchSegment's error case (facebook#46854)

Summary:
Pull Request resolved: facebook#46854

When segment fetch fails, it seems incorrect to call the callback twice: once with error, and once with null.

This seems like a bug.

Created from CodeHub with https://fburl.com/edit-in-codehub

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D63959015

fbshipit-source-id: f3ebb0f4732d99d91b5717a5bf193f44d957bde4

packages/react-native/Libraries/Core/setUpSegmentFetcher.js

@@ -42,6 +42,7 @@ function __fetchSegment(
         const error = new Error(errorObject.message);
         (error: any).code = errorObject.code; // flowlint-line unclear-type: off
         callback(error);
+        return;
       }
 
       callback(null);