Fix: add gcc -Og check and fix its false positive warnings

[t-mat]

Before adding gcc-12 and -Og test to our CI, I'd like to fix gcc -Og issue.
See also #810

Actually, this is a false positve of gcc -Og. So there must be another ways to avoid this warning.
But since it seems there's easy / safe / explicit fix for it with negilible const, I'd like to it to XXH3_INITSTATE().
( See advice #810 (comment) from @easyaspi314 )

[t-mat]

note: Test badge shows red ❌ , but it actually means "some tests are skipped". #817 fixes this issue.