Fixed uncaught BadWindow in EWMH check.

Fixes #234.

README.md

@@ -59,6 +59,8 @@ The following dependencies are needed by the examples and test programs:
  - [X11] Bugfix: The case of finding no usable CRTCs was not detected
  - [X11] Bugfix: Detection of broken Nvidia RandR gamma support did not verify
                  that at least one CRTC was present
+ - [X11] Bugfix: A stale `_NET_SUPPORTING_WM_CHECK` root window property would
+                 cause an uncaught `BadWindow` error
 
 
 ## Contact

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)
     {