Merge branch 'jk/misc-resolve-ref-unsafe-fixes'

Some codepaths did not check for errors when asking what branch the
HEAD points at, which have been fixed.

* jk/misc-resolve-ref-unsafe-fixes:
  worktree: handle broken symrefs in find_shared_symref()
  log: handle broken HEAD in decoration check
  remote: handle broken symrefs
  test-ref-store: avoid passing NULL to printf