add_invalidatee: simplify, placate static analyser
add_invalidatee: simplify, placate static analyser

Previously the Clang Static Analyzer thought we could end up with n = 0
after this. It shouldn't be possible, with space->invalidatees always
being either null or containing at least one non-null element and
null-terminated. Might as well clarify that in the code.

Deewiant/mushspace,Deewiant/mushspace,Deewiant/mushspace