ipv6: Check ip6_find_1stfragopt() return value properly.

Do not use unsigned variables to see if it returns a negative
error or not.

Fixes: 2423496a

 ("ipv6: Prevent overrun when parsing v6 header options")
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>