patch 8.1.0555: crash when last search pat is set but not last substitute pat

Problem:    Crash when last search pat is set but not last substitute pat.
Solution:   Do not mix up last search pattern and last subtitute pattern.
            (closes #3647)
diff --git a/src/version.c b/src/version.c
index 6cce566..566f94d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    555,
+/**/
     554,
 /**/
     553,