FIXUP: gatt: Fix potential invalid memory access

Fix invalid read:

==765== Invalid read of size 4
==765==    at 0x80D0932: read_request_result (gatt.c:2207)
==765==    by 0x80CFE9E: read_name_cb (gatt.c:1879)
==765==    by 0x80D0CB9: read_request (gatt.c:2277)
==765==    by 0x80D17B7: channel_handler_cb (gatt.c:2605)
==765==    by 0x80A5B48: received_data (gattrib.c:422)
==765==    by 0x40BD17D: g_io_unix_dispatch (giounix.c:166)
==765==    by 0x407CD85: g_main_context_dispatch (gmain.c:2539)
==765==    by 0x407D124: g_main_context_iterate.isra.21 (gmain.c:3146)
==765==    by 0x407D56A: g_main_loop_run (gmain.c:3340)
==765==    by 0x41BB4D2: (below main) (libc-start.c:226)
==765==  Address 0x45dc828 is 0 bytes inside a block of size 8 free'd
==765==    at 0x402AF79: free (vg_replace_malloc.c:446)
==765==    by 0x4082D0A: standard_free (gmem.c:98)
==765==    by 0x80CFE9E: read_name_cb (gatt.c:1879)
==765==    by 0x80D0CB9: read_request (gatt.c:2277)
==765==    by 0x80D17B7: channel_handler_cb (gatt.c:2605)
==765==    by 0x80A5B48: received_data (gattrib.c:422)
==765==    by 0x40BD17D: g_io_unix_dispatch (giounix.c:166)
==765==    by 0x407CD85: g_main_context_dispatch (gmain.c:2539)
==765==    by 0x407D124: g_main_context_iterate.isra.21 (gmain.c:3146)
==765==    by 0x407D56A: g_main_loop_run (gmain.c:3340)
==765==    by 0x41BB4D2: (below main) (libc-start.c:226)
==765==