MDEV-24742 Server crashes in Charset::numchars / String::numchars

The crash happened because Item_aes_crypt::val_str() did not
set the character set of the result.

mysql-test/r/func_str.result

@@ -4835,3 +4835,15 @@ DROP TABLE t1;
 #
 # End of 10.1 tests
 #
+#
+# Start of 10.2 tests
+#
+#
+# MDEV-24742 Server crashes in Charset::numchars / String::numchars
+#
+SELECT NULL IN (RIGHT(AES_ENCRYPT('foo','bar'), LAST_INSERT_ID()), 'qux');
+NULL IN (RIGHT(AES_ENCRYPT('foo','bar'), LAST_INSERT_ID()), 'qux')
+NULL
+#
+# End of 10.2 tests
+#

mysql-test/t/func_str.test

@@ -1932,3 +1932,19 @@ DROP TABLE t1;
 --echo #
 --echo # End of 10.1 tests
 --echo #
+
+
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-24742 Server crashes in Charset::numchars / String::numchars
+--echo #
+
+SELECT NULL IN (RIGHT(AES_ENCRYPT('foo','bar'), LAST_INSERT_ID()), 'qux');
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

sql/item_strfunc.cc

@@ -338,6 +338,8 @@ String *Item_aes_crypt::val_str(String *str2)
                  rkey, AES_KEY_LENGTH / 8, 0, 0))
       {
         str2->length((uint) aes_length);
+        DBUG_ASSERT(collation.collation == &my_charset_bin);
+        str2->set_charset(&my_charset_bin);
         return str2;
       }
     }