patch 8.1.0300: the old window title might be freed twice

Problem:    The old window title might be freed twice. (Dominique Pelle)
Solution:   Do not free "oldtitle" in a signal handler but set a flag to have
            it freed later.
diff --git a/src/version.c b/src/version.c
index 7278799..e7ea55f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    300,
+/**/
     299,
 /**/
     298,