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/search.c b/src/search.c
index 793e042..de17595 100644
--- a/src/search.c
+++ b/src/search.c
@@ -496,7 +496,7 @@
 }
 
     void
-set_last_csearch(int c, char_u *s UNUSED, int len UNUSED)
+set_last_csearch(int c, char_u *s, int len)
 {
     *lastc = c;
     lastc_bytelen = len;
@@ -1789,7 +1789,7 @@
     }
     else		// repeat previous search
     {
-	if (*lastc == NUL && lastc_bytelen == 1)
+	if (*lastc == NUL && lastc_bytelen <= 1)
 	    return FAIL;
 	if (dir)	// repeat in opposite direction
 	    dir = -lastcdir;
@@ -1833,7 +1833,7 @@
 			return FAIL;
 		    col -= (*mb_head_off)(p, p + col - 1) + 1;
 		}
-		if (lastc_bytelen == 1)
+		if (lastc_bytelen <= 1)
 		{
 		    if (p[col] == c && stop)
 			break;
diff --git a/src/testdir/test_charsearch.vim b/src/testdir/test_charsearch.vim
index 142e6c8..9b7c9a0 100644
--- a/src/testdir/test_charsearch.vim
+++ b/src/testdir/test_charsearch.vim
@@ -38,6 +38,8 @@
   " clear the character search
   call setcharsearch({'char' : ''})
   call assert_equal('', getcharsearch().char)
+  call assert_beeps('normal ;')
+  call assert_beeps('normal ,')
 
   call assert_fails("call setcharsearch([])", 'E1206:')
   enew!
diff --git a/src/testdir/test_charsearch_utf8.vim b/src/testdir/test_charsearch_utf8.vim
index 82a807a..843edbb 100644
--- a/src/testdir/test_charsearch_utf8.vim
+++ b/src/testdir/test_charsearch_utf8.vim
@@ -13,6 +13,13 @@
   call assert_equal([0, 1, 43, 0], getpos('.'))
   normal! ,
   call assert_equal([0, 1, 28, 0], getpos('.'))
+  call assert_equal('最', getcharsearch().char)
+  call setcharsearch({'char' : ''})
+  call assert_equal('', getcharsearch().char)
+  call assert_beeps('normal ;')
+  call assert_equal([0, 1, 28, 0], getpos('.'))
+  call assert_beeps('normal ,')
+  call assert_equal([0, 1, 28, 0], getpos('.'))
   bw!
 endfunc
 
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,