BUG/MINOR: http-fetch: fix smp_fetch_hdr_ip()'s handling of brackets for IPv6

BUG/MINOR: http-fetch: fix smp_fetch_hdr_ip()'s handling of brackets for IPv6

IPv6 addresses can be read enclosed in brackets, but the length of the
string is not checked before checking them. If by lack of luck, the
buffer is empty but already contains '[' in the first place, we'd read
the byte at position -1, possibly crashing (even though in practice it
will not since allocated blocks will be precedeed by the malloc meta-
data). At least it could make asan/valgrind unhappy.

This can be backported to all versions.