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/util/stacktrace_threads.cpp:301: NULL_FIELD 175022 Reading field "_head", which is expected to possibly be "nullptr" in "this->_head" (checked 1 out of 1 times).
/src/mongo/util/stacktrace_threads.cpp:301: NULL_FIELD 175022 Assigning: "node" = "std::exchange(this->_head, node->intrusiveNext)".
/src/mongo/util/stacktrace_threads.cpp:302: NULL_FIELD 175022 Dereferencing "node", which is known to be "nullptr".