set display to null for race condition

https://github.com/dunst-project/dunst/blob/master/src/x11/x.c#L499
looks to be the source of some code which now sets display to NULL
for some type of race condition in X11. Xlibint.h allows for building
on Void Linux but needs min/max undefined. closes #1939

src/x11.cc

@@ -33,6 +33,9 @@
 #include "x11.h"
 
 #include <X11/X.h>
+#include <X11/Xlibint.h>
+#undef min
+#undef max
 #include <sys/types.h>
 
 #include "common.h"
@@ -314,6 +317,8 @@ void update_x11_resource_db(bool first_run) {
       XrmDestroyDatabase(db);
     }
 
+    // https://github.com/dunst-project/dunst/blob/master/src/x11/x.c#L499
+    display->db = NULL; // should be new or deleted
     db = XrmGetStringDatabase((const char *)prop.value);
     XrmSetDatabase(display, db);
   }