Pointer to on-stack variable was returned from pending_write_new.

#0  0x00007ffff72047b0 in __memmove_ssse3 () from /usr/lib/libc.so.6
#1  0x00007ffff78ae440 in ?? () from /usr/lib/libdbus-1.so.3
#2  0x00007ffff78ad7cc in ?? () from /usr/lib/libdbus-1.so.3
#3  0x00007ffff789ac46 in ?? () from /usr/lib/libdbus-1.so.3
#4  0x00000000004720e8 in write_setup_cb (iter=0x7fffffffe710,
user_data=0x76c2a0) at src/gatt-database.c:1516
#5  0x00000000004b7b4c in g_dbus_proxy_method_call (proxy=0x75ade0,
method=0x4e1747 "WriteValue", setup=0x472088 <write_setup_cb>,
    function=0x4720fd <write_reply_cb>, user_data=0x76c2a0,
destroy=0x471f28 <pending_op_free>) at gdbus/client.c:875
#6  0x000000000047235f in send_write (attrib=0x764e00, proxy=0x75ade0,
owner_queue=0x764250, id=1, value=0x767ff3 "\001\377\177", len=1)
    at src/gatt-database.c:1597
#7  0x0000000000472bdf in chrc_write_cb (attrib=0x764e00, id=1,
offset=0, value=0x767ff3 "\001\377\177", len=1, opcode=82 'R',
att=0x7606d0,
    user_data=0x7641d0) at src/gatt-database.c:1865
#8  0x00000000004c96f2 in gatt_db_attribute_write (attrib=0x764e00,
offset=0, value=0x767ff3 "\001\377\177", len=1, opcode=82 'R',
att=0x7606d0,
    func=0x4c53c8 <write_complete_cb>, user_data=0x772320) at
src/shared/gatt-db.c:1570
#9  0x00000000004c5609 in write_cb (opcode=82 'R', pdu=0x767ff1,
length=3, user_data=0x7630e0) at src/shared/gatt-server.c:796
#10 0x00000000004bdb35 in handle_notify (att=0x7606d0, opcode=82 'R',
pdu=0x767ff1 "\f", pdu_len=3) at src/shared/att.c:768
#11 0x00000000004bddc9 in can_read_data (io=0x7607a0,
user_data=0x7606d0) at src/shared/att.c:849
#12 0x00000000004c9c44 in watch_callback (channel=0x7607d0,
cond=G_IO_IN, user_data=0x760940) at src/shared/io-glib.c:170
#13 0x00007ffff7b1662d in g_main_context_dispatch () from
/usr/lib/libglib-2.0.so.0
#14 0x00007ffff7b16a08 in ?? () from /usr/lib/libglib-2.0.so.0
#15 0x00007ffff7b16d32 in g_main_loop_run () from
/usr/lib/libglib-2.0.so.0
#16 0x0000000000466ba1 in main (argc=1, argv=0x7fffffffec38) at
src/main.c:661

This patch add support to cancel prepare write done from gatt client

Note that Write Execute 0x00 is done on cancel

This patch makes sure that long write is not proceeded when prepare
write has been called. Instead long write commands will be queued and
once write execute command is done, queued long write requests will
be proceeded.

It does not work in other way. Meaning, when long write is ongoing,
prepare write will not be proceeded nor queued. This feature can be
added later on if really needed.

For Android we need explicit write prepare and execute commands in GATT
client. This patch adds that.

Note that till now prepare and execute has been used only in long
write.

With this patch it is possible to start reliable write session by
the gatt-client. First write prepare will return session ID which shall
be used in follow up write prepare request and write execute.

Note that write execute is mapped to write execute 0x01
Following patch will add support to send write execute 0x00 using
bt_gatt_client_cancel

Clear in_long_write flag when cancel write execute is done