buffer: slow buffer copy compatibility fix

Fix issue where SlowBuffers couldn't be passed as target to Buffer
copy().

Also included checks to see if Argument parameters are defined before
assigning their values. This offered ~3x's performance gain.

Backport of 16bbecc from master branch. Closes #4633.

lib/buffer.js

@@ -545,8 +545,8 @@ Buffer.prototype.copy = function(target, target_start, start, end) {
     end = target.length - target_start + start;
   }
 
-  return this.parent.copy(target.parent,
-                          target_start + target.offset,
+  return this.parent.copy(target.parent || target,
+                          target_start + (target.offset || 0),
                           start + this.offset,
                           end + this.offset);
 };

src/node_buffer.cc

@@ -412,10 +412,10 @@ Handle<Value> Buffer::Copy(const Arguments &args) {
   Local<Object> target = args[0]->ToObject();
   char* target_data = Buffer::Data(target);
   size_t target_length = Buffer::Length(target);
-  size_t target_start = args[1]->Uint32Value();
-  size_t source_start = args[2]->Uint32Value();
-  size_t source_end = args[3]->IsUint32() ? args[3]->Uint32Value()
-                                          : source->length_;
+  size_t target_start = args[1]->IsUndefined() ? 0 : args[1]->Uint32Value();
+  size_t source_start = args[2]->IsUndefined() ? 0 : args[2]->Uint32Value();
+  size_t source_end = args[3]->IsUndefined() ? source->length_
+                                              : args[3]->Uint32Value();
 
   if (source_end < source_start) {
     return ThrowException(Exception::Error(String::New(

test/simple/test-buffer.js

@@ -79,6 +79,14 @@ for (var i = 0; i < c.length; i++) {
   assert.equal(i % 256, c[i]);
 }
 
+// copy from fast to slow buffer
+var sb = new SlowBuffer(b.length);
+var copied = b.copy(sb);
+console.log('copied %d bytes from b into sb');
+for (var i = 0; i < sb.length; i++) {
+  assert.strictEqual(sb[i], b[i]);
+}
+
 var caught_error = null;
 
 // try to copy from before the beginning of b