stream-restore: Fix a potential crash in pa_namereg_is_valid_name()

stream-restore: Fix a potential crash in pa_namereg_is_valid_name()

pa_namereg_is_valid_name() will hit an assertion if the name string is
NULL. Maybe it would make sense to change pa_namereg_is_valid_name() so
that it would return false on NULL, but I didn't want to change the
function semantics at this time.

e->device and e->card can be NULL even when device_valid and card_valid
are set to true if the database contains bad data.

I ran into this crash while developing new code, I haven't seen the
crash in the wild.