patch 9.0.1555: setcharsearch() does not clear last searched char properly

Problem:    setcharsearch() does not clear last searched char properly.
Solution:   Do not accept lastc_bytelen smaller than one. (closes #12398)
diff --git a/src/version.c b/src/version.c
index 8620f97..292f228 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1555,
+/**/
     1554,
 /**/
     1553,