Uninitialized memory used in buf_flush_insert_into_flush_list

mtr --valgrind innodb.innodb_bug21704

==29405== Uninitialised byte(s) found during client check request
==29405== at 0x124891A: buf_flush_insert_into_flush_list(buf_pool_t*, buf_block_t*, unsigned long) (buf0flu.cc:434)
==29405== by 0x105F16C: buf_flush_note_modification(buf_block_t*, unsigned long, unsigned long, FlushObserver*) (buf0flu.ic:72)
==29405== by 0x1061DAD: ReleaseBlocks::add_dirty_page_to_flush_list(mtr_memo_slot_t*) const (mtr0mtr.cc:378)
==29405== by 0x1061DFC: ReleaseBlocks::operator()(mtr_memo_slot_t*) const (mtr0mtr.cc:390)
==29405== by 0x1062920: CIterate<ReleaseBlocks const>::operator()(mtr_buf_t::block_t*) const (mtr0mtr.cc:60)
==29405== by 0x106240D: bool mtr_buf_t::for_each_block_in_reverse<CIterate<ReleaseBlocks const> >(CIterate<ReleaseBlocks const> const&) const (dyn0buf.h:371)
==29405== by 0x10610C4: mtr_t::Command::release_blocks() (mtr0mtr.cc:882)
==29405== by 0x10611CD: mtr_t::Command::execute() (mtr0mtr.cc:910)
==29405== by 0x105FD75: mtr_t::commit() (mtr0mtr.cc:558)
==29405== by 0x118F2E8: trx_purge_truncate_rseg_history(trx_rseg_t&, purge_sys_t::iterator const&) (trx0purge.cc:493)
==29405== by 0x118F6D6: trx_purge_truncate_history() (trx0purge.cc:563)
==29405== by 0x1191D10: trx_purge(unsigned long, bool) (trx0purge.cc:1322)
==29405== by 0x116684B: srv_do_purge(unsigned long*) (srv0srv.cc:2623)
==29405== by 0x1166C82: srv_purge_coordinator_thread (srv0srv.cc:2749)
==29405== by 0x4E42568: start_thread (in /lib64/libpthread-2.26.so)
==29405== by 0x6ECBA2E: clone (in /lib64/libc-2.26.so)
==29405== Address 0x112e0434 is 1,076 bytes inside a frame of size 16,384 client-defined
==29405== at 0x1227BB3: buf_block_init(buf_pool_t*, buf_block_t*, unsigned char*) (buf0buf.cc:1511)
==29405== by 0x1228044: buf_chunk_init(buf_pool_t*, buf_chunk_t*, unsigned long) (buf0buf.cc:1657)
==29405== by 0x1228AE9: buf_pool_init_instance(buf_pool_t*, unsigned long, unsigned long) (buf0buf.cc:1891)
==29405== by 0x12294FD: buf_pool_init(unsigned long, unsigned long) (buf0buf.cc:2087)
==29405== by 0x116C476: srv_start(bool) (srv0start.cc:1512)