Don't crash on torrc Vi[rtualAddrNetworkIPv[4|6]] with no option value

Check for a missing option value in parse_virtual_addr_network
before asserting on the NULL in tor_addr_parse_mask_ports.
This avoids crashing on torrc lines like Vi[rtualAddrNetworkIPv[4|6]]
when no value follows the option.

Bugfix on 0.2.3 (de4cc126cbb5 on 24 November 2012), fixes #14142.

changes/bug14142-parse-virtual-addr

@@ -0,0 +1,7 @@
+  o Minor bugfixes (client):
+    - Check for a missing option value in parse_virtual_addr_network
+      before asserting on the NULL in tor_addr_parse_mask_ports.
+      This avoids crashing on torrc lines like
+      Vi[rtualAddrNetworkIPv[4|6]] when no value follows the option.
+      Bugfix on 0.2.3 (de4cc126cbb5 on 24 November 2012), fixes #14142.
+      Patch by "teor".

src/or/addressmap.c

@@ -738,6 +738,12 @@ parse_virtual_addr_network(const char *val, sa_family_t family,
   const int max_bits = ipv6 ? 40 : 16;
   virtual_addr_conf_t *conf = ipv6 ? &virtaddr_conf_ipv6 : &virtaddr_conf_ipv4;
 
+  if (!val || val[0] == '\0') {
+    if (msg)
+      tor_asprintf(msg, "Value not present (%s) after VirtualAddressNetwork%s",
+                   val?"Empty":"NULL", ipv6?"IPv6":"");
+    return -1;
+  }
   if (tor_addr_parse_mask_ports(val, 0, &addr, &bits, NULL, NULL) < 0) {
     if (msg)
       tor_asprintf(msg, "Error parsing VirtualAddressNetwork%s %s",