fix an old bug where dd_frm_type() could incorrectly determine the table type

for dynamic engines (because it only looked at the one-byte code, not at the full
engine name).

mysql-test/t/func_str.test

@@ -6,6 +6,8 @@
 drop table if exists t1,t2;
 --enable_warnings
 
+let $mysqld_datadir= `select @@datadir`;
+
 set names latin1;
 
 select 'hello',"'hello'",'""hello""','''h''e''l''l''o''',"hel""lo",'hel\'lo';
@@ -1385,6 +1387,8 @@ LOAD DATA INFILE 'bug58165.txt' INTO TABLE t1;
 SELECT * FROM t1;
 DROP TABLE t1;
 
+--remove_file $mysqld_datadir/test/bug58165.txt
+
 #
 # MDEV-759 lp:998340 - Valgrind complains on simple selects containing expression DAY(FROM_UNIXTIME(-1))
 #