Asserts when encountering invalid files

The reported entry point is /usr/bin/dlopen-notes, especially read_dlopen_notes and the argument loop around line 142. Start by tracing how filenames are opened and how invalid, missing, and directory inputs reach ELFFile. Add coverage for those inputs and verify the command avoids assertions as requested.

Version: 0.11-1
systemd: 256.9-1
kernel: 6.12.4-arch1-1

While writing a script to find all dl-opened libraries on a system, I ran into this assert when encountering .py files:

Traceback (most recent call last):
  File "/usr/bin/dlopen-notes", line 142, in <module>
    notes = [read_dlopen_notes(filename) for filename in args.filenames]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/bin/dlopen-notes", line 24, in wrap
    return list(f(*args, **kwargs))
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/bin/dlopen-notes", line 29, in read_dlopen_notes
    elffile = ELFFile(open(filename, 'rb'))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/elftools/elf/elffile.py", line 84, in __init__
    self._identify_file()
  File "/usr/lib/python3.12/site-packages/elftools/elf/elffile.py", line 570, in _identify_file
    elf_assert(magic == b'\x7fELF', 'Magic number does not match')
  File "/usr/lib/python3.12/site-packages/elftools/common/utils.py", line 80, in elf_assert
    _assert_with_exception(cond, msg, ELFError)
  File "/usr/lib/python3.12/site-packages/elftools/common/utils.py", line 143, in _assert_with_exception
    raise exception_type(msg)
elftools.common.exceptions.ELFError: Magic number does not match

Ideally, invalid & non-existent files (& directories) wouldn't cause asserts like this.