[X86] Fix MCNullStreamer support for modules with a CodeView flag

This fixes -filetype=null support when compiling for a Win32 target and the module has a CodeView flag.

The only places changed are the uses of getTargetStreamer function - this patch guards both of them with null checks.

Committed on behalf of @eush (Eugene Sharygin)

Differential Revision: https://reviews.llvm.org/D54008

llvm-svn: 346962