Fix handling of structure for bytea data type in ECPG

Fix handling of structure for bytea data type in ECPG

Some code paths dedicated to bytea used the structure for varchar.  This
did not lead to any actual bugs, as bytea and varchar have the same
definition, but it could become a trap if one of these definitions
changes for a new feature or a bug fix.

Issue introduced by 050710b.

Author: Shenhao Wang
Reviewed-by: Vignesh C, Michael Paquier
Discussion: https://postgr.es/m/07ac7dee1efc44f99d7f53a074420177@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 12