python: Fix check for disasm creation failure

Check should be for pointer being NULL, not what it points to.

Also adds a test for this case.

Reviewed By: indygreg

Differential Revision: http://llvm-reviews.chandlerc.com/D1878



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194965 91177308-0d34-0410-b5e6-96231b3b80d8