Potential crash in update checker

[nielsm]

Missing error checking in the update checker can cause crashes.

In dialog_version_check.cpp, AegisubVersionCheckerThread::DoCheck(), the following line:

std::auto_ptr<wxInputStream> stream(http.GetInputStream(path));

No check is made whether http.GetInputStream() returns 0, which might cause crashes later.

[nielsm]

Crash log pointing to the problem

[nielsm]

(In [4383]) Add a null-pointer check to update checker. Updates #1164.

[nielsm]

(In [4384]) Merge r4383, closes #1164.