Fix memory leak in LibreSSL/tls_conninfo_free()

[Shuo Chen]

Valgrind finds out that conninfo->servername is not free()d by
tls_conninfo_free().

== HEAP SUMMARY:
==     in use at exit: 83,069 bytes in 2,690 blocks
==   total heap usage: 4,107 allocs, 1,417 frees,
==                     339,660 bytes allocated
==
== 17 bytes in 1 blocks are definitely lost in loss record 1 of 266
==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==    by 0x58F5989: strdup (strdup.c:42)
==    by 0x40B2C4: tls_conninfo_populate
==    by 0x408C4F: tls_handshake
==    by 0x403691: TlsContext::handshake()
==    by 0x403343: TlsStream::connect(TlsConfig*, char const*,
==    by 0x407781: main (in /home/schen/recipes/ssl/client)
==
== LEAK SUMMARY:
==    definitely lost: 17 bytes in 1 blocks
==    indirectly lost: 0 bytes in 0 blocks
==      possibly lost: 0 bytes in 0 blocks
==    still reachable: 83,052 bytes in 2,689 blocks
==         suppressed: 0 bytes in 0 blocks

Here's a quick fix.

Regards,
Shuo Chen

====================================================
diff --git a/tls/tls_conninfo.c b/tls/tls_conninfo.c
--- a/tls/tls_conninfo.c
+++ b/tls/tls_conninfo.c
@@ -248,6 +248,8 @@ tls_conninfo_free(struct tls_conninfo *conninfo)
        conninfo->alpn = NULL;
        free(conninfo->cipher);
        conninfo->cipher = NULL;
+       free(conninfo->servername);
+       conninfo->servername = NULL;
        free(conninfo->version);
        conninfo->version = NULL;