Fix: handle unhandled exceptions

Hi all!

I was working with poco-1.4.6 but I see this bug was also in previous versions.
Stable version of poco-1.4.6 works well on my Ubuntu-12.04_x64-86 but it works wrong on Windows 8.1 x64.

I am using Poco::Net::HTTPSStreamFactory. Sample:

Poco::Net::HTTPSStreamFactory httpsFactory;
std::istream* pIStream = httpsFactory.open( some_url );

This code should download some data and it does it but... with additional extra zero-byte in the end of these data.
After investigation I found that istream's implementation on Linux differs from Windows 8.1 one. Both handle exceptions that can be inside of istream::get(). But in Linux in this case EOF will be returned and in Windows 8.1 we get zero.

So I offer this patch.

Thanks,
Oleg

@obiltschnig @aleks-f