CJamRecorder: Add currentSession pointer initialization and improve locking

[cdmahoney]

• Added initialization of currentSession pointer in CJamRecorder constructor
• Changed locking code in CJamRecorder to use QMutexLock, to prevent leaving mutex locked if method exits with exception or nested return statement.

Server code tested on Windows and Ubuntu 20.04.

[ann0see]

Thanks for opening this PR!

@pljones should have a look at this code since he wrote most of it.

[ann0see]

Jamulus uses an odd code style. You should apply clang format ;-).

[hoffie]

Thanks!
On a high level, this looks good to me. I'm leaving the more extensive reviews for @pljones and @softins though.

There's one formatting thing to fix (you can run clang-format on it if you have version 10 or later). I've just approved the workflows for this PR which will probably also show the relevant line.

[softins]

Looks fine to me, subject to the formatting adjustments that have already been mentioned. Thanks!

[pljones]

Looks fine to me.

[cdmahoney]

OK, will upload corrected file later - after running clang-format! Thanks to all for the helpful comments.

[ann0see]

Not tested personally; code style seems to be ok. I'll let @pljones or someone else merge it.

[hoffie]

CHANGELOG: Jam Recorder's internal locking and initiatlization code has been made more robust.

[pljones]

Now running on my jam server.