[D-runtime] win32 regression in exception handling

[Brad Roberts]

Now that the dust of the svn -> git migration has settled some, there's a failure that's stuck with us.  Since this
druntime change, the win32 dmd test34.d test has failed:

https://github.com/D-Programming-Language/druntime/commit/559df80b3d9c51272c5804de8df27af75099b977

http://d.puremagic.com/test-results/test_data.ghtml?dataid=43441

  Before test 3:
  object.Error at src\rt\deh.d(631): Access Violation

I did a test build at the previous commit and the test passes.  So that's definitely the one that introduces the
problem.  It might well be just exposing a dormant problem, but either way, it stopped working at that point.