buffer: standardize array index check

ParseArrayIndex() was requesting a Uint32Value(), but assigning it to an
in32_t. This caused slight differences in error message reported in edge
cases of argument parsing. Fixed by getting the IntegerValue() before
checking if the value is < 0. Added test of API that was affected.

PR-URL: https://github.com/nodejs/node/pull/6084
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_internals.h

@@ -169,7 +169,7 @@ inline MUST_USE_RESULT bool ParseArrayIndex(v8::Local<v8::Value> arg,
     return true;
   }
 
-  int32_t tmp_i = arg->Uint32Value();
+  int64_t tmp_i = arg->IntegerValue();
 
   if (tmp_i < 0)
     return false;

test/parallel/test-buffer-alloc.js

@@ -1440,3 +1440,9 @@ assert.equal(Buffer.prototype.parent, undefined);
 assert.equal(Buffer.prototype.offset, undefined);
 assert.equal(SlowBuffer.prototype.parent, undefined);
 assert.equal(SlowBuffer.prototype.offset, undefined);
+
+
+// Test that ParseArrayIndex handles full uint32
+assert.throws(function() {
+  Buffer.from(new ArrayBuffer(0), -1 >>> 0);
+}, /RangeError: 'offset' is out of bounds/);