MDEV-23600 follow-up: uninitialized rec_field_is_prefix

build_template_field(): Initialize templ->rec_field_is_prefix
also for indexes on virtual columns. This was caught on 10.5 by
MemorySanitizer as use-of-uninitialized-value in
row_search_with_covering_prefix() when running the test
main.fast_prefix_index_fetch_innodb.

storage/innobase/handler/ha_innodb.cc

@@ -7455,6 +7455,8 @@ build_template_field(
 #ifdef HAVE_valgrind_or_MSAN
 	MEM_UNDEFINED(templ, sizeof *templ);
 #endif /* HAVE_valgrind_or_MSAN */
+	templ->rec_field_is_prefix = FALSE;
+	templ->rec_prefix_field_no = ULINT_UNDEFINED;
 	templ->is_virtual = !field->stored_in_db();
 
 	if (!templ->is_virtual) {
@@ -7516,8 +7518,6 @@ build_template_field(
 				<< " query "
 				<< innobase_get_stmt_unsafe(current_thd, &size);
 		}
-		templ->rec_field_is_prefix = FALSE;
-		templ->rec_prefix_field_no = ULINT_UNDEFINED;
 
 		if (dict_index_is_clust(index)) {
 			templ->rec_field_no = templ->clust_rec_field_no;
@@ -7535,7 +7535,6 @@ build_template_field(
 		DBUG_ASSERT(!ha_innobase::omits_virtual_cols(*table->s));
 		col = &dict_table_get_nth_v_col(index->table, v_no)->m_col;
 		templ->clust_rec_field_no = v_no;
-		templ->rec_prefix_field_no = ULINT_UNDEFINED;
 
 		if (dict_index_is_clust(index)) {
 			templ->rec_field_no = templ->clust_rec_field_no;