CLEANUP: ncbuf: use standard BUG_ON with DEBUG_STRICT

CLEANUP: ncbuf: use standard BUG_ON with DEBUG_STRICT

ncbuf can be compiled for haproxy or standalone to run unit test suite.
For the latest mode, BUG_ON() macro has been re-implemented in a simple
version.

The inclusion of the default or the redefined macro relied on DEBUG_DEV.
Change this to now rely on DEBUG_STRICT as this is activated for the
default build.

This change is safe as only BUG_ON_HOT() macro is used in ncbuf code,
which is activated only with the default value DEBUG_STRICT=2.

This should be backported up to 2.6.