Closes https://github.com/MusicPlayerDaemon/MPD/issues/1285

Without clearing the "uri" field, the next Open() call attempts to
reuse the old InputStream, but it has already been closed, so Open()
always returns nullptr.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1300

Almost all callers have string literal, and the length is known at
compile time.

The inverted check was introduced by commit 46d00dd8, and commit
8ad17d25 added a check for the wrong variable.  D'oh!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1303

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1297

MPD supports only 8 channels, so MAX_CHANNELS is enough, the array
doens't need to be SPA_AUDIO_MAX_CHANNELS (which is 64).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1297

.. instead of DeferEvent, which is not thread-safe.  This caused
various playback problems, which was initially caused by the
DeferEvent/InjectEvent split in commit 774b4313

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1298

Currently fails in class NfsFileReader due to
https://github.com/MusicPlayerDaemon/MPD/issues/1298