Check return code from malloc (#20173)

Summary:
Calls abort() in cases where malloc returns NULL.

Checking the return value from malloc is good practice and is
required to pass a [Veracode security scan](https://www.veracode.com/). This will let
developers who are required to submit their software to Veracode
use React Native.
Pull Request resolved: https://github.com/facebook/react-native/pull/20173

Differential Revision: D9235096

Pulled By: hramos

fbshipit-source-id: 9fdc97f9e84f8d4d91ae59242093907f7a81d286

Libraries/Network/RCTNetworking.mm

@@ -50,6 +50,12 @@ static NSString *RCTGenerateFormBoundary()
   const char *boundaryChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_.";
 
   char *bytes = (char*)malloc(boundaryLength);
+  if (!bytes) {
+    // CWE - 391 : Unchecked error condition
+    // https://www.cvedetails.com/cwe-details/391/Unchecked-Error-Condition.html
+    // https://eli.thegreenplace.net/2009/10/30/handling-out-of-memory-conditions-in-c
+    abort();
+  }
   size_t charCount = strlen(boundaryChars);
   for (int i = 0; i < boundaryLength; i++) {
     bytes[i] = boundaryChars[arc4random_uniform((u_int32_t)charCount)];