Malformed filter can cause crash

[raxod502]

% utunes read '' -f album:example
Traceback (most recent call last):
  File "/home/raxod502/files/control/data/emacs-repos/utunes/utunes/__init__.py", line 485, in main
    field, regex = filter_str.split("=")
ValueError: not enough values to unpack (expected 2, got 1)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/raxod502/files/control/data/emacs-repos/utunes/utunes/__main__.py", line 3, in <module>
    utunes.main()
  File "/home/raxod502/files/control/data/emacs-repos/utunes/utunes/__init__.py", line 487, in main
    parser_read.error("malformed filter string: {}".format(repr(field)))
UnboundLocalError: local variable 'field' referenced before assignment