Fix render issue when button has conditional

If the button has a conditional, the image render function will
return a 0 which indicated that an image was present even if it
was not if the condition was not being met during boot. When the
condition is met, mButtonImg was not set to NULL causing a render
request failure.

Change-Id: I32be0ddcdc1bf7ad22440d66018ef09484280afa
diff --git a/gui/button.cpp b/gui/button.cpp
index 3f9c2df..be52c27 100644
--- a/gui/button.cpp
+++ b/gui/button.cpp
@@ -58,15 +58,19 @@
 
 	if (!node)  return;
 
-	// Three of the four can be loaded directly from the node
-	mButtonImg = new GUIImage(node);
+	// These can be loaded directly from the node
 	mButtonLabel = new GUIText(node);
 	mAction = new GUIAction(node);
 
-	if (mButtonImg->Render() < 0)
+	child = node->first_node("image");
+	if (child)
 	{
-		delete mButtonImg;
-		mButtonImg = NULL;
+		mButtonImg = new GUIImage(node);
+		if (mButtonImg->Render() < 0)
+		{
+			delete mButtonImg;
+			mButtonImg = NULL;
+		}
 	}
 	if (mButtonLabel->Render() < 0)
 	{