[PRISM] Fix crash when --parser=prism called with stdin

[Bug #20071]

Currently Ruby crashes when the --parser=prism flag is used either with
no input, or with input that is being redirected from stdin. So all of
the following will crash

ruby --parser=prism
ruby --parser=prism < test_code.rb
cat test_code.rb | ruby --parser=prism

This commit checks whether the input is assumed to be from stdin, and
then processes that as a file.

This will fix the second and third case above, but will cause a slight
behavioural changes for the first case - Ruby will treat stdin as an
empty file in this case and exit, rather than waiting for data to be
piped into stdin.

0 files changed, 0 insertions, 0 deletions