tun/tap: Fix crashes if open() /dev/net/tun and then poll() it.

Fix NULL pointer dereference in tun_chr_pool() introduced by commit
33dccbb0 ("tun: Limit amount of queued
packets per device") and triggered by this code:

	int fd;
	struct pollfd pfd;
	fd = open("/dev/net/tun", O_RDWR);
	pfd.fd = fd;
	pfd.events = POLLIN | POLLOUT;
	poll(&pfd, 1, 0);

Reported-by: Eugene Kapun <abacabadabacaba@gmail.com>
Signed-off-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>