The `held == 0` branch in the Lck__AssertHeld function is *never*
taken and `cache.h` instructs to only use the macro. Being generated
from calling code, the panic message becomes more informative.

Panic messages were up until now not very helpful:

    Assert error in Lck__Assert(), cache/cache_lck.c line 175:
    Condition(ilck->held) not true.