In get_bc_algorithm_name, add a dummy return statement.

This code shouldn't be reached, but having it here might avoid a
compiler warning.

Per CI complaint from Andres Freund.

Discussion: http://postgr.es/m/C6A7643A-582B-47F7-A03D-01736BC0349B@anarazel.de

src/common/backup_compression.c

@@ -72,6 +72,7 @@ get_bc_algorithm_name(bc_algorithm algorithm)
 			/* no default, to provoke compiler warnings if values are added */
 	}
 	Assert(false);
+	return "???";	/* placate compiler */
 }
 
 /*