[pcp] pcp updates: libpcp: plug memory leak in derived metrics error handling

[Mark Goodwin]

QA passes for group 'derive'. Frank's valgrind repro in BZ#1331973 also
now passes - at this stage of the release, this should be reviewed prior
to merging. I'll also create a new QA test for the repro script.
	
Changes committed to git://git.pcp.io/mgoodwin/pcp master

commit 188987bde837f6f81c259903ab08824bcd008214
Author: Mark Goodwin <mgoodwin at redhat.com>
Date:   Wed Jun 15 17:54:45 2016 +1000

     libpcp: plug memory leak in derived metrics error handling
     
     BZ #1331973
     
     Error handling needs to recursively free the current node, since it
     may have been built recursively, so call free_expr() instead of free()
     in the appropriate places where a derived expression fails. The patch
     also fortifies free_expr() itself a bit.
     
     Passes qa for group 'derive', and Frank's valgrind repro script passes
     now too. A new QA test should probably be added.
     
     BTW, this is not a regression per-se - the leak has always been there -
     it's just more noticeable now that global derived metrics defs are loaded
     by default.