Bug#51675 Server crashes on inserting 4 byte char. after ALTER TABLE to 'utf8mb4'

Bug#51676 Server crashes on SELECT, ORDER BY on 'utf8mb4' column
An additional fix. We should use 0xFFFD as a weight for supplementary
characters, not the "weight for character U+FFFD".

mysql-test/r/ctype_utf8mb4.result

@@ -2456,16 +2456,16 @@ t1	CREATE TABLE `t1` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 SELECT *  FROM t1 ORDER BY 1;
 subject	p
-𐐀	NULL
 abc𐐀def	NULL
+𐐀	NULL
 SELECT hex(subject), length(subject), char_length(subject), octet_length(subject) FROM t1 ORDER BY 1;
 hex(subject)	length(subject)	char_length(subject)	octet_length(subject)
 616263F0909080646566	10	7	10
 F0909080	4	1	4
 SELECT subject FROM t1 ORDER BY 1;
 subject
-𐐀
 abc𐐀def
+𐐀
 DROP TABLE t1;
 #
 # End of 5.5 tests

strings/ctype-uca.c

@@ -6983,11 +6983,18 @@ static int my_uca_scanner_next_any(my_uca_scanner *scanner)
                                           scanner->send)) <= 0))
       return -1;
     
-    if (wc > 0xFFFF)
-      wc= MY_CS_REPLACEMENT_CHARACTER;
-    scanner->page= wc >> 8;
-    scanner->code= wc & 0xFF;
     scanner->sbeg+= mb_len;
+    if (wc > 0xFFFF)
+    {
+      /* Return 0xFFFD as weight for all characters outside BMP */
+      scanner->wbeg= nochar;
+      return 0xFFFD;
+    }
+    else
+    {
+      scanner->page= wc >> 8;
+      scanner->code= wc & 0xFF;
+    }
     
     if (scanner->contractions && !scanner->page &&
         (scanner->code > 0x40) && (scanner->code < 0x80))