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 050710b3.

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

src/interfaces/ecpg/ecpglib/data.c

@@ -523,8 +523,8 @@ ecpg_get_data(const PGresult *results, int act_tuple, int act_field, int lineno,
 
 				case ECPGt_bytea:
 					{
-						struct ECPGgeneric_varchar *variable =
-						(struct ECPGgeneric_varchar *) (var + offset * act_tuple);
+						struct ECPGgeneric_bytea *variable =
+						(struct ECPGgeneric_bytea *) (var + offset * act_tuple);
 						long		dst_size,
 									src_size,
 									dec_size;

src/interfaces/ecpg/ecpglib/descriptor.c

@@ -591,8 +591,8 @@ set_desc_attr(struct descriptor_item *desc_item, struct variable *var,
 
 	else
 	{
-		struct ECPGgeneric_varchar *variable =
-		(struct ECPGgeneric_varchar *) (var->value);
+		struct ECPGgeneric_bytea *variable =
+		(struct ECPGgeneric_bytea *) (var->value);
 
 		desc_item->is_binary = true;
 		desc_item->data_len = variable->len;

src/interfaces/ecpg/ecpglib/execute.c

@@ -822,8 +822,8 @@ ecpg_store_input(const int lineno, const bool force_indicator, const struct vari
 
 			case ECPGt_bytea:
 				{
-					struct ECPGgeneric_varchar *variable =
-					(struct ECPGgeneric_varchar *) (var->value);
+					struct ECPGgeneric_bytea *variable =
+					(struct ECPGgeneric_bytea *) (var->value);
 
 					if (!(mallocedval = (char *) ecpg_alloc(variable->len, lineno)))
 						return false;
@@ -1401,7 +1401,7 @@ ecpg_build_params(struct statement *stmt)
 
 			if (var->type == ECPGt_bytea)
 			{
-				binary_length = ((struct ECPGgeneric_varchar *) (var->value))->len;
+				binary_length = ((struct ECPGgeneric_bytea *) (var->value))->len;
 				binary_format = true;
 			}
 		}