[obj2yaml] Stop parsing the debug_str section when it encounters a string...

[obj2yaml] Stop parsing the debug_str section when it encounters a string without the null terminator.

When obj2yaml encounters a string without the null terminator, it should
stop parsing the debug_str section. This patch addresses comments in
[D86867](https://reviews.llvm.org/D86867#inline-803291).

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D87261