core: deregister xerror handler in session_destroy

xerror handler might be called during XCloseDisplay, at which point
session_t is already destroyed, causing a segfault.

Ping #209

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/common.h

@@ -153,6 +153,8 @@ typedef struct session {
 	// === Display related ===
 	/// Display in use.
 	Display *dpy;
+	/// Previous handler of X errors
+	XErrorHandler previous_xerror_handler;
 	/// Default screen.
 	int scr;
 	/// XCB connection.