Error returned by UPnPContext::getAvailablePortNumber not handled correctly

The UPnPContext::getAvailablePortNumber function returns 0 if it can't find an available port number, but this case isn't handled correctly in the following two functions:

• UPnPContext::provisionNewMappings simply calls getAvailablePortNumber again whenever this happens, which can lead to an infinite loop;
• UPnPContext::registerMapping doesn't check whether the port number it got is zero or not, so it will go ahead and request a port mapping with internal and external port numbers set to 0, which is invalid.