• Remove ancient checks for sim->softc == NULL. It can't be NULL, as it is

set not-NULL during SIM registration and set to UMASS_GONE on destruction.
Debug messages there look broken for at least 9 years, as they dereference
softc value that was just checked to be equal to NULL.

• Remove magic pointer value UMASS_GONE and use simple NULL instead.

Found by: Clang Static Analyzer