Skip invalid mid in Sweeper::clearMultis

[hannahbast]

Not sure if this is an appropriate fix, but spatialjoin then runs through without errors and the results look reasonable.

[patrickbr]

Thanks, @hannahbast. An invalid mid should never happen here. I strongly suspect a missing lock for the _subOverlaps maps is the cause here. The mid here comes directly from the event file - it is either an offset into the cache file (for normal events) or an offset into the multi-geometry ID vector. An mid of 130033639634376183 is obviously not a valid offset into the file, as we do not have 130 PB of cache data. So memory corruption must be the cause, and this was most like caused by concurrent read/write access to _subOverlaps invalidating all iterators in multiOut(). After that, we are in the purgatory of undefined behavior :)