netfilter: nf_conntrack_sip: fix the ct_sip_parse_numerical_param() return value.

ct_sip_parse_numerical_param() returns only 0 or 1 now.
But process_register_request() and process_register_response() imply
checking for a negative value if parsing of a numerical header parameter
failed.
The invocation in nf_nat_sip() looks correct:
 	if (ct_sip_parse_numerical_param(...) > 0 &&
 	    ...) { ... }

Make the return value of the function ct_sip_parse_numerical_param()
a tristate to fix all the cases
a) return 1 if value is found; *val is set
b) return 0 if value is not found; *val is unchanged
c) return -1 on error; *val is undefined

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with SVACE.

Fixes: 0f32a40f ("[NETFILTER]: nf_conntrack_sip: create signalling expectations")
Signed-off-by: Ilia.Gavrilov <Ilia.Gavrilov@infotecs.ru>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>