previously fctx_done() detached the fctx but did not clear the pointer
passed into it from the caller. in some conditions, when rctx_done()
was reached while waiting for a validator to complete, fctx_done()
could be called twice on the same fetch, causing a double detach.
fctx_done() now clears the fctx pointer, to reduce the chances of
such mistakes.
(cherry picked from commit b4592d02a1)
8a90a62d0f