DB ADOdb MetaIndex fails when no constraints exist

Working with Oracle database (oci8), when MetaIndex() method is called
to retrieve the list of indexes for a table that has no constraints
set, an error is triggered:
Fatal error: Call to a member function FetchRow() on a non-object

This commit adds a check to cleanly exit the function, returning false.

Similar error handling has also been added to the calling database_api
function db_index_exists()

This allows the DB installation (schema.php) to proceed without errors.

Fixes #13438

Porting to 1.3 - Conflicts:
	library/adodb/drivers/adodb-oci8.inc.php

Note: the fix to ADOdb driver does not need to be ported as it was
included in a later release of the library.

core/database_api.php

@@ -626,11 +626,13 @@ function db_index_exists( $p_table_name, $p_index_name ) {
 
 	if( is_blank( $p_index_name ) || is_blank( $p_table_name ) ) {
 		return false;
-
-		// no index found
 	}
 
 	$t_indexes = $g_db->MetaIndexes( $p_table_name );
+	if( $t_indexes === false ) {
+		// no index found
+		return false;
+	}
 
 	if( !empty( $t_indexes ) ) {
 		# Can't use in_array() since it is case sensitive