patch 8.1.1857: cannot use modifier with multi-byte character

Problem:    Cannot use modifier with multi-byte character.
Solution:   Allow using a multi-byte character, although it doesn't work
            everywhere.
diff --git a/src/misc2.c b/src/misc2.c
index 463b37b..db3687d 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -2819,10 +2819,10 @@
 		    l = mb_ptr2len(bp + 1);
 		else
 		    l = 1;
-		/* Anything accepted, like <C-?>.
-		 * <C-"> or <M-"> are not special in strings as " is
-		 * the string delimiter. With a backslash it works: <M-\"> */
-		if (!(in_string && bp[1] == '"') && bp[2] == '>')
+		// Anything accepted, like <C-?>.
+		// <C-"> or <M-"> are not special in strings as " is
+		// the string delimiter. With a backslash it works: <M-\">
+		if (!(in_string && bp[1] == '"') && bp[l + 1] == '>')
 		    bp += l;
 		else if (in_string && bp[1] == '\\' && bp[2] == '"'
 							       && bp[3] == '>')