BUILD: bug: make BUG_ON() void to avoid a rare warning

BUILD: bug: make BUG_ON() void to avoid a rare warning

When building without threads, the recently introduced BUG_ON(tid != 0)
turns to a constant expression that evaluates to 0 and that is not used,
resulting in this warning:

  src/connection.c: In function 'conn_free':
  src/connection.c:584:3: warning: statement with no effect [-Wunused-value]

This is because the whole thing is declared as an expression for clarity.
Make it return void to avoid this. No backport is needed.