COMP: Fix buffer lookahead for wrapper-parser include directive.

Commit 8977cc9c did not properly handle the 2-character lookahead
required for handling backslashes while preprocessing included files.
It would sometimes duplicate the character following the backslash.

Backport of fd79beff from master to release.

Change-Id: I554c2adf507989364b49b2732dea8388190f35f1