BUG/MINOR: cfgparse-listen: release the previous description of the proxy

BUG/MINOR: cfgparse-listen: release the previous description of the proxy

The proxy's "description" keyword did not release the previous value
before assigning the new one, so the first one was leaked when the
keyword was used twice, e.g. once in a defaults section and once in the
proxy. Note that the global section's "description" does it properly.

This may be backported to all supported versions.