bug in heredoc parser wrt apostrophes

Trying to use `mrsh` as shell to run [`mksh/Build.sh`](http://www.mirbsd.org/cvs.cgi/src/bin/mksh/Build.sh?rev=HEAD) with fails.

MWE:

if true; then
echo one
else
cat <<'EOF'
/* apo'stroph */
EOF
#'
fi

Fails with:

syntax error: expected 'fi'