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)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1555,
 /**/
     1554,
 /**/