af_key: fix buffer overread in parse_exthdrs()

If a message sent to a PF_KEY socket ended with an incomplete extension
header (fewer than 4 bytes remaining), then parse_exthdrs() read past
the end of the message, into uninitialized memory.  Fix it by returning
-EINVAL in this case.

Reproducer:

	#include <linux/pfkeyv2.h>
	#include <sys/socket.h>
	#include <unistd.h>

	int main()
	{
		int sock = socket(PF_KEY, SOCK_RAW, PF_KEY_V2);
		char buf[17] = { 0 };
		struct sadb_msg *msg = (void *)buf;

		msg->sadb_msg_version = PF_KEY_V2;
		msg->sadb_msg_type = SADB_DELETE;
		msg->sadb_msg_len = 2;

		write(sock, buf, 17);
	}

Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>