playlist/preparser: cancellation leads to partial art download

If the thread responsible for fetching art is cancelled while art is still being downloaded, the cache will be populated with an entry containing partial content.

An easy way to reproduce the issue is by doing something as the below:

 % cat /tmp/foo.m3u
 #EXTM3U
 #EXTALBUMARTURL:http://localhost:1337/korv.png
 file:///tmp/sample_1.mp3

 % echo -e \
 > "HTTP/1.0 200 OK\r\n"\
 > "Content-Type: image/png\r\n"\
 > "Content-Length: 1337\r\n"\
 > "\r\n"\
 > "this is spaa-" | nc -l -p 1337

Then:

Play /tmp/foo.m3u
Notice that VLC tries to download the art from our fake httpd
Close VLC

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information