Fixed uncaught BadWindow in EWMH check.

Fixes #234.

src/x11_init.c

@@ -359,6 +359,8 @@ static void detectEWMH(void)
         return;
     }
 
+    _glfwGrabXErrorHandler();
+
     // It should be the ID of a child window (of the root)
     // Then we look for the same property on the child window
     if (_glfwGetWindowProperty(*windowFromRoot,
@@ -371,6 +373,10 @@ static void detectEWMH(void)
         return;
     }
 
+    _glfwReleaseXErrorHandler();
+    if (_glfw.x11.errorCode != Success)
+        return;
+
     // It should be the ID of that same child window
     if (*windowFromRoot != *windowFromChild)
     {