Fix crash on exit if pages have been closed (#12482)

Regression from #12431: the loop to clear all surviving pages
neglects to check for entries in `m_pages` which have already
been closed, fully destructed, and therefore replaced by NULL
pointers.
run-jasmine example: Ensure exit code is always returned.

https://github.com/ariya/phantomjs/pull/12486
Disable colorized output in tests on Windows.

https://github.com/ariya/phantomjs/issues/12484