MythMusic: fix playback of some radio stations

Make sure the maximum size of the QTCPSocket read buffer is set appropriately.

This seems to throttle the sending of readyRead() signals sent to us from
QTCPSocket better otherwise once we fill our buffer and don't read anything in the
readyRead() slot the QTCPSocket will eventually not send any more signals causing
us to run out of data.

Fixes #11872.

(cherry picked from commit 6ddaba216d8f6a7e178d4acfe6930458fb532d05)
MetaIOAVFComment: don't call av_estimate_timings() in getTrackLength()

It's not needed with current ffmpeg and was causing a segfault while scanning
for music files.

(cherry picked from commit c2128934644892df613590dda7e6017b72c14f94)
MetaIO: validate the files extension before creating a tagger

This checks the file extension is one we know how to handle before
creating the tagger. Return NULL if not valid.

This fixes a bug with the music file scanner trying to add files with an
unknown file extension.

NOTE: bumps the ABI version so compile and install myth core before compiling
the plugins

(cherry picked from commit 47217ca10adae1e03a37b396d64b605c8f18e6b0)
Bump the ABI version after f359a4ebd3c
MythMusic: make sure we null check the returned tagger after 47217ca10a

Also make sure the tagger is deleted after use.

(cherry picked from commit b383c329417a4e4b0b786e51e0d24962728a738b)
MythMusic: avoid storing a pointer to the current playlist in the music player

Always get the current playlist from MusicData to avoid possibly referencing a
stale pointer. This can occur while doing a rescan.

Also add NULL checks everywhere just in case.

Refs #11906.

(cherry picked from commit 193dc636e5fa5ac5edd3589094a58e79a6ab779b)