patch 8.2.5113: timer becomes invalid after fork/exec, :gui gives errors

Problem:    Timer becomes invalid after fork/exec, :gui gives errors. (Gabriel
            Dupras)
Solution:   Delete the timer befor forking. (closes #10584)

src/gui.c

@@ -226,6 +226,11 @@ gui_do_fork(void)
     int		exit_status;
     pid_t	pid = -1;
 
+#if defined(FEAT_RELTIME) && defined(HAVE_TIMER_CREATE)
+    // a timer is not carried forward
+    delete_timer();
+#endif
+
     // Setup a pipe between the child and the parent, so that the parent
     // knows when the child has done the setsid() call and is allowed to
     // exit.