pmwebapi: adapt to some libmicrohttpd's multiple "completed" callbacks
pmwebapi: adapt to some libmicrohttpd's multiple "completed" callbacks

In some circumstances, we could receive two callbacks, and would try
to delete the postprocessor/context objects each time.  Now, clear the
state variable after delete so we don't try it again.

Tested by preexisting 660 test case, failing with libmicrohttpd 0.9.34-4.fc22.x86_64

mbaldessari/pcp,mbaldessari/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,wuliming/pcp,mbaldessari/pcp,tjanez/pcp,prasincs/pcp,edwardt/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,aeg-aeg/pcpfans,prasincs/pcp,edwardt/pcp,andyvand/cygpcpfans,adfernandes/pcp,edwardt/pcp,aeg-aeg/pcpfans,edwardt/pcp,andyvand/cygpcpfans,wuliming/pcp,prasincs/pcp,wuliming/pcp,prasincs/pcp,adfernandes/pcp,prasincs/pcp,wuliming/pcp,edwardt/pcp,aeg-aeg/pcpfans,adfernandes/pcp,wuliming/pcp,aeg-aeg/pcpfans,tjanez/pcp,mbaldessari/pcp,adfernandes/pcp,tjanez/pcp,wuliming/pcp,edwardt/pcp,tjanez/pcp,edwardt/pcp,tjanez/pcp,prasincs/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,andyvand/cygpcpfans,wuliming/pcp,andyvand/cygpcpfans,tjanez/pcp,adfernandes/pcp,adfernandes/pcp,edwardt/pcp,adfernandes/pcp,mbaldessari/pcp,tjanez/pcp,andyvand/cygpcpfans,prasincs/pcp,tjanez/pcp,aeg-aeg/pcpfans,mbaldessari/pcp,wuliming/pcp,adfernandes/pcp,prasincs/pcp