Fixed broken WGL error detection.

Context creation was attempted even if no valid pixel formats had been
found during enumeration.

src/wgl_context.c

@@ -276,9 +276,13 @@ static GLboolean choosePixelFormat(_GLFWwindow* window,
     }
 
     closest = _glfwChooseFBConfig(desired, usableConfigs, usableCount);
-    if (closest)
-        *result = closest->wgl;
+    if (!closest)
+    {
+        free(usableConfigs);
+        return GL_FALSE;
+    }
 
+    *result = closest->wgl;
     free(usableConfigs);
 
     return GL_TRUE;