patch 8.2.5126: substitute may overrun destination buffer

Problem:    Substitute may overrun destination buffer.
Solution:   Disallow switching buffers in a substitute expression.
diff --git a/src/version.c b/src/version.c
index 82ac4ea..2f397ae 100644
--- a/src/version.c
+++ b/src/version.c
@@ -735,6 +735,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    5126,
+/**/
     5125,
 /**/
     5124,