sim: Initialize pbb_br_* by default

On the files generated by sim/common/genmloop.sh, variables pbb_br_type and
pbb_br_npc are declared uninitialized and passed to other functions in some
cases.  Despite that those are harmless, they will generate GCC warnings
("-Wmaybe-uninitialized").

This commit ensures that pbb_br_type and pbb_br_npc variables are
initialized to a harmless value.