Silence compiler warnings on macOS

[bradhowes]

• Don't compare unsigned int values as NULL
• Hoist variable declarations to top of function
• Migrate from deprecated identifier

[derselbst]

LGTM in general, thanks. Pls. have a look at the failing build on Mac 10.15. And two nit-picks below.

[bradhowes]

@derselbst Thanks for the reviews. I checked out the failures below and they all involve pipeline setup failures:

• Timeouts fetching mingw-64 (I was able to manually fetch but it is slow).
• Chocolatey failure due to missing checksums for the pkgconfiglite package -- looks like it can be fixed with -allow-empty-checksums flag or manually calculate a checksum and pass it to the Chocolatey command line.

[derselbst]

Yeah, the Windows checks are a bit unstable. The Mac builds are green now, which matter most. Thank you! If you want you can rebase your changes against the 2.2.x branch. Otherwise I would merge it manually by tomorrow or so, so we can ship this with fluidsynth 2.2.6.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[derselbst]

Cherry-picked to 2.2.x and merged back to master. Thanks!