t-reftable-block: release used block reader

Used block readers must be released using block_reader_release() to
prevent the occurence of a memory leak. Make test_block_read_write()
conform to this statement.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/t-reftable-block.c

@@ -105,6 +105,7 @@ static void t_block_read_write(void)
 		block_iter_close(&it);
 	}
 
+	block_reader_release(&br);
 	reftable_record_release(&rec);
 	reftable_block_done(&br.block);
 	strbuf_release(&want);