In AllocGroupSmallMap::find(AllocGroup) we were calling lower_bound(...) and
then unconditionally dereferencing the resulting iterator, however
lower_bound(...) may return end() if the value being searched for is higher
than any value present in the map. This patch adds a check for end() before
the dereference to guard against dereference of end().
This commit also adds some basic unit tests for MemProt and AllocGroupSmallMap.
rdar://129662981
