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: 0f32a40fc91a ("[NETFILTER]: nf_conntrack_sip: create signalling expectations")
Signed-off-by: Ilia.Gavrilov <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Reviewed-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_conntrack_sip.c

···
 	start += strlen(name);
 	*val = simple_strtoul(start, &end, 0);
 	if (start == end)
-		return 0;
+		return -1;
 	if (matchoff && matchlen) {
 		*matchoff = start - dptr;
 		*matchlen = end - start;