m: Ignore mutex poisoning in X11_BACKEND

A panic doesn't really put any of the fields in XConnection into an invalid
state, so there is no real reason to panic when poisoning is detected.
So just ignore the poison.

Closes #3870

Signed-off-by: John Nunley <dev@notgull.net>

src/platform_impl/linux/mod.rs

@@ -626,7 +626,7 @@ unsafe extern "C" fn x_error_callback(
     display: *mut x11::ffi::Display,
     event: *mut x11::ffi::XErrorEvent,
 ) -> c_int {
-    let xconn_lock = X11_BACKEND.lock().unwrap();
+    let xconn_lock = X11_BACKEND.lock().unwrap_or_else(|e| e.into_inner());
     if let Ok(ref xconn) = *xconn_lock {
         // Call all the hooks.
         let mut error_handled = false;
@@ -748,7 +748,7 @@ impl EventLoop {
 
     #[cfg(x11_platform)]
     fn new_x11_any_thread() -> Result<EventLoop, EventLoopError> {
-        let xconn = match X11_BACKEND.lock().unwrap().as_ref() {
+        let xconn = match X11_BACKEND.lock().unwrap_or_else(|e| e.into_inner()).as_ref() {
             Ok(xconn) => xconn.clone(),
             Err(_) => return Err(EventLoopError::NotSupported(NotSupportedError::new())),
         };