Avoid segfault with malformed IPv6 address

A literal IPv6 address without a trailing bracket will result in a write
past the end of the address buffer:

~~~ segfault.conf
protocols:
(
 { name: "tls"; host: "["; port: "8443"; }
);
~~~

~~~
$ sslh-select -p 127.0.0.1:443 --foreground -F./segfault.conf
[: no closing bracket in IPv6 address?
Segmentation fault (core dumped)
~~~

common.c

@@ -496,6 +496,7 @@ int resolve_split_name(struct addrinfo **out, const char* ct_host, const char* s
        end = strrchr(host, ']');
        if (!end) {
            fprintf(stderr, "%s: no closing bracket in IPv6 address?\n", host);
+           return -1;
        }
        host++; /* skip first bracket */
        *end = 0; /* remove last bracket */