Fix .seh_stackalloc 0

seh_stackalloc 0 is not representable in Win64 SEH info, so emitting it
is a bug.

Reviewers: rnk

Differential Revision: http://reviews.llvm.org/D4334

Patch by Vadim Chugunov!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212081 91177308-0d34-0410-b5e6-96231b3b80d8