Fix use-after-close lazy IO bug

Make `getPrologue` force `parseParas dflags str` before returning. Without this,
it will attempt to read from the file after it is closed, with unspecified and
generally bad results.

Signed-off-by: David Feuer <David.Feuer@gmail.com>
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>