NFC: Fix possible NULL ptr deref when getting the name of a socket

llcp_sock_getname() might get called before the LLCP socket was created.
This condition isn't checked, and llcp_sock_getname will simply deref a
NULL ptr in that case.

This exists starting with d646960 ("NFC: Initial LLCP support").

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>