MDEV-24335 Unexpected question mark in the end of a TINYTEXT column

my_copy_fix_mb() passed MIN(src_length,dst_length) to
my_append_fix_badly_formed_tail(). It could break a multi-byte
character in the middle, which put the question mark to the
destination.

Fixing the code to pass the true src_length to
my_append_fix_badly_formed_tail().

mysql-test/r/ctype_utf8.result

@@ -11256,5 +11256,16 @@ DROP TABLE kv;
 DROP VIEW v1;
 SET NAMES utf8;
 #
+# MDEV-24335 Unexpected question mark in the end of a TINYTEXT column
+#
+CREATE TABLE t1 (a TINYTEXT character set utf8);
+INSERT IGNORE INTO t1 VALUES (REPEAT(_utf8 0xD184, 250));
+Warnings:
+Warning	1366	Incorrect string value: '\xD1\x84\xD1\x84\xD1\x84...' for column `test`.`t1`.`a` at row 1
+SELECT LENGTH(a), CHAR_LENGTH(a), RIGHT(a,3) FROM t1;
+LENGTH(a)	CHAR_LENGTH(a)	RIGHT(a,3)
+254	127	ффф
+DROP TABLE t1;
+#
 # End of 10.2 tests
 #

mysql-test/t/ctype_utf8.test

@@ -2183,6 +2183,15 @@ DROP TABLE kv;
 DROP VIEW v1;
 SET NAMES utf8;
 
+--echo #
+--echo # MDEV-24335 Unexpected question mark in the end of a TINYTEXT column
+--echo #
+
+CREATE TABLE t1 (a TINYTEXT character set utf8);
+INSERT IGNORE INTO t1 VALUES (REPEAT(_utf8 0xD184, 250));
+SELECT LENGTH(a), CHAR_LENGTH(a), RIGHT(a,3) FROM t1;
+DROP TABLE t1;
+
 --echo #
 --echo # End of 10.2 tests
 --echo #

strings/ctype-mb.c

@@ -401,10 +401,10 @@ my_copy_fix_mb(CHARSET_INFO *cs,
   size_t well_formed_nchars;
   size_t well_formed_length;
   size_t fixed_length;
+  size_t min_length= MY_MIN(src_length, dst_length);
 
-  set_if_smaller(src_length, dst_length);
   well_formed_nchars= cs->cset->well_formed_char_length(cs,
-                                                        src, src + src_length,
+                                                        src, src + min_length,
                                                         nchars, status);
   DBUG_ASSERT(well_formed_nchars <= nchars);
   well_formed_length= status->m_source_end_pos - src;