Fix M3U parser truncating EXTINF duration to single character (#3152)

The EXTINF duration parser used `info[0].strip()[0]` which takes only
the first character of the duration string. For multi-digit durations
like "120", this truncates to "1". Also, the `-1` check on line 79
compared against "-1" but could only ever see "-" (a single char),
so negative/unknown durations were never properly detected.

Remove the erroneous `[0]` index so the full duration string is used
and the `-1` sentinel is correctly recognized.

Co-authored-by: David Bishop <[email protected]>
Co-authored-by: Claude Opus 4.6 <[email protected]>