Issue 384 - UnregisterWait error handling is incorrect

http://code.google.com/p/google-breakpad/issues/detail?id=384

ClientInfo::UnregisterWaits calls UnregisterWait (Windows API), and checks the
return value. But there is a trick of the return value of UnregisterWait:
http://msdn.microsoft.com/en-us/library/ms686870%28VS.85%29.aspx says that
if there is a callback pending on the handle, the function will unregister
the wait, return FALSE and GetLastError will have ERROR_IO_PENDING.

This means that the ClientInfo ends up with a invalid handle. When we get
to ClientInfo::~ClientInfo we try to UnregisterWaitEx again, which crashes
with an INVALID_PARAMETER exception.

[doshimun]

LGTM. Good catch. I think some product at Google is also seeing a similar crash.
Thanks for fixing it.