Fix a regression on the disassembling C API.

The fix is easy. Unfortunately, we had 0 tests, so adding one was somewhat
complicated.

Thanks to Kevin Enderby for the report.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221899 91177308-0d34-0410-b5e6-96231b3b80d8