If fifo_open() is called with a negative file descriptor, return EINVAL

rather than panicking later.  This can occur if the kernel calls
vn_open() on a fifo, as there will be no associated file descriptor,
and therefore the file descriptor operations cannot be modified to
point to the fifo operation set.

MFC after:	3 days
Reported by:	Martin <nakal at nurfuerspam dot de>
PR:		94278

sys/fs/fifofs/fifo_vnops.c

@@ -179,6 +179,8 @@ fifo_open(ap)
 	int error;
 
 	ASSERT_VOP_LOCKED(vp, "fifo_open");
+	if (ap->a_fdidx < 0)
+		return (EINVAL);
 	if ((fip = vp->v_fifoinfo) == NULL) {
 		MALLOC(fip, struct fifoinfo *, sizeof(*fip), M_VNODE, M_WAITOK);
 		error = socreate(AF_LOCAL, &rso, SOCK_STREAM, 0, cred, td);