Dereference of potentially null field

If the field is actually null, then a null pointer dereference will occur. Field, which is statistically checked against null, is dereferenced.
/src/mongo/base/shim.h:120: NULL_FIELD 175094 Reading field "f", which is expected to possibly be "nullptr" in "this->_slot->f" (checked 1 out of 1 times).