[windows] Fix out-of-memory failure in some of the tests

Summary: When ReadProcessMemory fails, bytes_read is sometimes set to a large garbage value. In that case, we need to set it back to zero before returning or the garbage value will be used to allocate memory later causing LLDB to crash with an out of memory error.

Reviewers: asmith, zturner

Reviewed By: zturner

Subscribers: zturner, asmith, stella.stamenova, llvm-commits

Differential Revision: https://reviews.llvm.org/D49159

llvm-svn: 336865