wireguard: add local variable

wireguard: add local variable

The current implementation of wireguard use dereference value from
pointer, but between get and dereference the value from pointer can be
occur change in pool memory, which means that this pointer can be
invalid. Since current implementation doesn't handle with invalid
pointers, segfault can occur.

The fix add a local variable to keep index of peer from pool and also
handle with null pointers from get pointer from pool.

Type: fix
Signed-off-by: Gabriel Oginski <gabrielx.oginski@intel.com>
Change-Id: Ic161ab08266e584493338c682d827ea1fd754b98