[FIX] IndexError when parsing incomplete urls without a host part

[mib112]

When parsing an incomplete url without host like "http://" a IndexError will be thrown. In contrast to this the upstream javascript version returns an almost empty parse result:

url {protocol: 'http:', slashes: true, auth: null, port: null, hostname: '', pathname: null, search: null, hash: null}

The IndexError occurs in Python on accessing an array with a negative index. In contrast, in javascript a negative index results in the value "undefined".

See also executablebooks/markdown-it-py#205

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Codecov Report

Merging #7 (6542655) into master (5c2b361) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #7   +/-   ##
=======================================
  Coverage   77.02%   77.02%           
=======================================
  Files           6        6           
  Lines         296      296           
=======================================
  Hits          228      228           
  Misses         68       68           

Impacted Files	Coverage Δ
src/mdurl/_parse.py	97.85% <100.00%> (ø)

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