Read within bounds of `NSData` object

Summary: This prevents the `isUnbundle` check to read beyond the end of an `NSData` instance by using `getBytes:length:` instead of accessing the underlying buffer directly.

Reviewed By: javache

Differential Revision: D3548874

fbshipit-source-id: 7c93c66cc6abb4a2a321888ab394212f4d14a03e

React/Executors/RCTJSCExecutor.mm

@@ -730,8 +730,9 @@ static NSData *loadPossiblyBundledApplicationScript(NSData *script, NSURL *sourc
                                                     NSError **error)
 {
   // The RAM bundle has a magic number in the 4 first bytes `(0xFB0BD1E5)`.
-  uint32_t magicNumber = NSSwapLittleIntToHost(*((uint32_t *)script.bytes));
-  isRAMBundle = magicNumber == RCTRAMBundleMagicNumber;
+  uint32_t magicNumber = 0;
+  [script getBytes:&magicNumber length:sizeof(magicNumber)];
+  isRAMBundle = NSSwapLittleIntToHost(magicNumber) == RCTRAMBundleMagicNumber;
   if (isRAMBundle) {
     [performanceLogger markStartForTag:RCTPLRAMBundleLoad];
     script = loadRAMBundle(sourceURL, error, randomAccessBundle);