Fix crash in StrictSubstr on -ohos artifacts

[jschwe]

Fixes a "region out-of-bounds" assertion failure in StrictSubstr when running bloaty on a Rust shared library compiled for aarch64-unknown-linux-ohos.

When remaining_ is just the "\0" character, we are done and should return.

[EricRahm]

@jschwe Thanks for the proposed fix! I rebased and tested this and it looks like it regresses a few tests:

Failed Tests (5):
  bloaty :: dwarf/debug_info/dw_form_line_strp.test
  bloaty :: dwarf/debug_info/ignore-declarations.test
  bloaty :: dwarf/debug_info/pcpair-incomplete.test
  bloaty :: dwarf/debug_info/pcpair.test
  bloaty :: dwarf/range_lists/base-addr-selection.test

I'm guessing the fix is a bit more nuanced. It might be worth filing an issue including a binary that reproduces to track this.

If you're up for it we'd want to add a lit test to reproduce the issue and a rebased and updated fix.