Fix make_y_.m for purely forward/backward models.

This commit is linked to 052faa2f93643a15ea1a94218209c74e753ee29a and issue #613.
(cherry picked from commit 86dd4a56120b61ed8baa3a306050b8cb8c789d1a)
Fix size of M_.endo_histval.

Should fix crashes on models with leads/lags of more than one on exogenous.

Closes #617
(cherry picked from commit 9ba5814c2c475e27c8afe21c9ecd5d31c36e2eea)