Change pfree to accept NULL argument

Per discussion in [0], here is a patch set that allows pfree() to accept 
a NULL argument, like free() does.

Also, a patch that removes the now-unnecessary null pointer checks 
before calling pfree().  And a few patches that do the same for some 
other functions that I found around.  (The one with FreeDir() is perhaps 
a bit arguable, since FreeDir() wraps closedir() which does *not* accept 
NULL arguments.  Also, neither FreeFile() nor the underlying fclose() 
accept NULL.)


[0]: https://www.postgresql.org/message-id/1074830.1655442689@sss.pgh.pa.us