Notice a little faster if we're running out of virtual addresses

We were not decrementing "available" every time we did
++next_virtual_addr in addressmap_get_virtual_address: we left out the
--available when we skipped .00 and .255 addresses.

This didn't actually cause a bug in most cases, since the failure mode
was to keep looping around the virtual addresses until we found one,
or until available hit zero.  It could have given you an infinite loop
rather than a useful message, however, if you said "VirtualAddrNetwork
127.0.0.255/32" or something broken like that.

Spotted by cypherpunks

src/or/connection_edge.c

@@ -1169,6 +1169,10 @@ addressmap_get_virtual_address(int type)
       while ((next_virtual_addr & 0xff) == 0 ||
              (next_virtual_addr & 0xff) == 0xff) {
         ++next_virtual_addr;
+        if (! --available) {
+          log_warn(LD_CONFIG, "Ran out of virtual addresses!");
+          return NULL;
+        }
       }
       in.s_addr = htonl(next_virtual_addr);
       tor_inet_ntoa(&in, buf, sizeof(buf));