[WK1] NPStream::headers not initialized

https://bugs.webkit.org/show_bug.cgi?id=119574

Reviewed by Alexey Proskuryakov.

Initialize the header field. According to user feedback that should solve some
cases of random crashes when loading plugins on Mac and Windows.

* plugins/PluginStream.cpp:
(WebCore::PluginStream::PluginStream):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@153828 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Task-number: QTBUG-32181
Change-Id: I45410e618cf0337ac1b2c8c4bafa572e8e1e0d19
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>