Fix memory leak in postmasterMain

Hi

This is fairly inconsequential as memory leaks goes, but if -D is used 
when starting postgres, the memory allocated by stdrup() will never be 
freed. Found with valgrind.

Technically, this also happens with output_config_variable (-C), but since 
postmaster prints and exits with that, there isn't much point in fixing 
it.


   best regards, Henrik