shared/gatt-server: Fix crash on read multiple

shared/gatt-server: Fix crash on read multiple

When read multiple includes external characteristic, call to
gatt_db_attribute_read's complete callback is asynchronous.

Fix following crash:
Use of uninitialised value of size 8
   at 0x49718A: read_multiple_complete_cb (gatt-server.c:994)
   by 0x498999: pending_read_result (gatt-db.c:136)
   by 0x49A84B: gatt_db_attribute_read_result (gatt-db.c:1787)
   by 0x451F4C: read_reply_cb (gatt-database.c:1712)
   by 0x48B221: method_call_reply (client.c:972)
   by 0x53AB601: ??? (in /usr/lib64/libdbus-1.so.3.19.3)
   by 0x53AEF7E: dbus_connection_dispatch (in /usr/lib64/libdbus-1.so.3.19.3)
   by 0x486FEF: message_dispatch (mainloop.c:72)
   by 0x50CB576: ??? (in /usr/lib64/libglib-2.0.so.0.5400.3)
   by 0x50CEB76: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.5400.3)
   by 0x50CEF1F: ??? (in /usr/lib64/libglib-2.0.so.0.5400.3)
   by 0x50CF231: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.5400.3)

Invalid read of size 8
   at 0x49718A: read_multiple_complete_cb (gatt-server.c:994)
   by 0x498999: pending_read_result (gatt-db.c:136)
   by 0x49A84B: gatt_db_attribute_read_result (gatt-db.c:1787)
   by 0x451F4C: read_reply_cb (gatt-database.c:1712)
   by 0x48B221: method_call_reply (client.c:972)
   by 0x53AB601: ??? (in /usr/lib64/libdbus-1.so.3.19.3)
   by 0x53AEF7E: dbus_connection_dispatch (in /usr/lib64/libdbus-1.so.3.19.3)
   by 0x486FEF: message_dispatch (mainloop.c:72)
   by 0x50CB576: ??? (in /usr/lib64/libglib-2.0.so.0.5400.3)
   by 0x50CEB76: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.5400.3)
   by 0x50CEF1F: ??? (in /usr/lib64/libglib-2.0.so.0.5400.3)
   by 0x50CF231: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.5400.3)
 Address 0x8 is not stack'd, malloc'd or (recently) free'd

Change-Id: Id0aca942b2c5229e798c2d4fb9d9d8c3ac86cdde
Signed-off-by: Amit Purwar <amit.purwar@samsung.com>