adapter: Fix crash when storing link key

adapter: Fix crash when storing link key

The following trace can be observed sometimes when pairing 2 emulator
instances:

 src/adapter.c:store_link_key() Unable to load key file from
 /var/lib/bluetooth/9C:DA:3E:F2:8E:46/9C:B6:D0:8A:A0:0C/info: (No
such file or directory)
 GLib: g_file_set_contents: assertion 'error == NULL ||
*error == NULL' failed
 ++++++++ backtrace ++++++++
 #1  btd_backtrace+0x28a (src/backtrace.c:59) [0x7f65bb5ab53a]
 #2  g_logv+0x21c (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.6400.6)
 [0x7f65ba3f955c]
 #3  g_log+0x93 (/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.6400.6)
 [0x7f65ba3f9743]
 #4  g_file_set_contents+0x68
(/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.6400.6) [0x7f65ba3dca68]
 #5  store_link_key+0x30a (src/adapter.c:8235) [0x7f65bb61839a]
 #6  new_link_key_callback+0x474 (src/adapter.c:8285) [0x7f65bb62c904]
 #7  queue_foreach+0x164 (src/shared/queue.c:203) [0x7f65bb722e34]
 #8  can_read_data+0x59f (src/shared/mgmt.c:343) [0x7f65bb72e09f]
 #9  watch_callback+0x112 (src/shared/io-glib.c:162) [0x7f65bb78acb2]
 #10 g_main_context_dispatch+0x14e
(/usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.6400.6) [0x7f65ba3f204e]

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>