Do not touch a NPStream object when it is already destructed.

BUG=20651
TEST=Run "sh tools/valgrind/valgrind_webkit_tests.sh --debug LayoutTests/plugins/destroy-stream-twice.html" and verify that the "Invalid read of size 4" error at NPP_DestroyStream() is not reported in the log (vlayout-NNNNN.log).

Review URL: http://codereview.chromium.org/354005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30769 0039d316-1c4b-4281-b951-d872f2087c98