gui: Initialize button render position

Change-Id: I716ae0664ea454706cb940d1bfd38259da53d176
diff --git a/gui/button.cpp b/gui/button.cpp
index 45614e4..a9b02a3 100644
--- a/gui/button.cpp
+++ b/gui/button.cpp
@@ -85,7 +85,7 @@
 
 	mHighlightColor = LoadAttrColor(FindNode(node, "highlight"), "color", &hasHighlightColor);
 
-	int x, y, w, h;
+	int x = 0, y = 0, w = 0, h = 0;
 	TextPlacement = TOP_LEFT;
 	if (mButtonImg) {
 		mButtonImg->GetRenderPos(x, y, w, h);