use_proxy(): Add missing terminating NUL byte

Fixes CVE-2025-9232

There is a missing terminating NUL byte after strncpy() call.
Issue and a proposed fix reported by Stanislav Fort (Aisle Research).

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

crypto/http/http_lib.c

@@ -263,6 +263,7 @@ static int use_proxy(const char *no_proxy, const char *server)
         /* strip leading '[' and trailing ']' from escaped IPv6 address */
         sl -= 2;
         strncpy(host, server + 1, sl);
+        host[sl] = '\0';
         server = host;
     }