D-Bus: Fix P2P NULL dereference after interface removal

D-Bus: Fix P2P NULL dereference after interface removal

When the P2P management interface is deleted, P2P is then disabled and
global->p2p_init_wpa_s is set to NULL. After that, other interfaces can
still trigger P2P functions (like wpas_p2p_find()) using D-Bus. This
makes wpa_supplicant terminate with SIGSEGV, because it dereferences a
NULL pointer. Fix this by adding proper checks, like it's done with
wpa_cli.

CC: Beniamino Galvani <bgalvani@redhat.com>
CC: Benjamin Berg <benjamin@sipsolutions.net>
Reported-by: Vladimir Benes <vbenes@redhat.com>
Signed-off-by: Davide Caratti <davide.caratti@gmail.com>