Avoid printing uninitialized filename variable in verbose mode

When using verbose mode for pg_basebackup, in tar format sent to
stdout, we'd print an unitialized buffer as the filename.

Reported by Pontus Lundkvist

src/bin/pg_basebackup/pg_basebackup.c

@@ -491,6 +491,7 @@ ReceiveTarFile(PGconn *conn, PGresult *res, int rownum)
 			else
 #endif
 				tarfile = stdout;
+			strcpy(filename, "-");
 		}
 		else
 		{