This small patch fixes the bug #17555. This increases the consistency
of `mark-paragraph` with `mark-word` and `mark-sexp`.

Currently "C-- M-h M-h" will end up selecting nothing since repeating
`mark-paragraph` always expands forwards even if `mark-paragraph` was
initially invoked with a negative prefix argument.

This is my first patch to Emacs. I have not signed the papers
assigning copyright to FSF. I would like to do so but I cannot figure
out how to.

Kind regards
Simon Friis Vindum