Add some probably unnecessary protection around pointers in mpeg2fix.cpp. None of these should cause a problem in normal operation, but the associated ticket appears to show the pointer to MPEG2fixup with a suspicious address which often sugests that it was unintialised. I believe the actual segfault may be due to one of the streams disappearing from the file and that requires more work to fix. Refs #12153

(cherry picked from commit ad1304f653aba0eb052a1f6b2415d94b24c0b9bb)
MythCoreContext: Fix bad copy/paste

(cherry picked from commit c5abaa12bcdc0aff444525ea386d6f8b44997992)
Allow to override which mpegts demuxer is used

Set FFMPEGTS override (call with -O FFMPEGTS=1)

Refs:#12153
(cherry picked from commit 1267856cfbcfc01d2ff5b7ed8ac116e44ef3a0c4)
Ignore audio packets from ignored streams

Fixes #12153

(cherry picked from commit 345e2d651fe6dcb3f14caec73854b1a386e52be9)
Add —allaudiotracks

By default mythtranscode exclude all audio tracks where the number of channels is detected as 0. However, under some circumstances our mpegts demuxer doesn’t properly detect the audio streams and mark them as having 0 channels.
Use —allaudiotracks to include all audio tracks found.

Refs: #12153
(cherry picked from commit ff36448c27359619130cb6e221659330b70f19c8)

Conflicts:
	mythtv/programs/mythtranscode/mpeg2fix.cpp
Don’t use reference to initially allocated avio buffer

aviocontext can resize the buffer under some circumstances. Attempting to later free the original buffer address would cause a crash

(cherry picked from commit b645d9c20525b695e11d5b1b909e845a5d09885c)
Increase size of streaming buffer

streaming buffer size is also used to detect audio format, 16kB may not be sufficient to properly detect corrupted media or FLAC ones.
The buffer will be resized to 32kB by FFmpeg once the file has been successfully opened

(cherry picked from commit fd1432712d30fa258b8a1118c87e6989b8af5571)
Let FFmpeg determine how to probe for format when using SG

This makes for much more reliable format detection.

(cherry picked from commit b7b160608926ec051ad51715cf5f24b6b5c7e1e4)