Bug 669236: Examine rv from is-Azure-enabled Preferences::GetBool() call, to fix "unused variable" build warning. r=bas

content/html/content/src/nsHTMLCanvasElement.cpp

@@ -733,8 +733,8 @@ NS_NewCanvasRenderingContext2D(nsIDOMCanvasRenderingContext2D** aResult)
   PRBool azure = PR_FALSE;
   nsresult rv = Preferences::GetBool("gfx.canvas.azure.enabled", &azure);
 
-  if (azure) {
-    nsresult rv = NS_NewCanvasRenderingContext2DAzure(aResult);
+  if (NS_SUCCEEDED(rv) && azure) {
+    rv = NS_NewCanvasRenderingContext2DAzure(aResult);
     // If Azure fails, fall back to a classic canvas.
     if (NS_SUCCEEDED(rv)) {
       return rv;