Fix ClientSocketPoolBaseHelper preconnect crasher.

It's caused by an invalid read that sometimes leads to an invalid write that causes a CHECK failure.

BUG=64985
TEST=New unit test added, fails under valgrind without the fix.

Review URL: http://codereview.chromium.org/5523001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@67942 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed