Replace ASSERT by null-check in InsertedNodes::lastLeafInserted()

Replace ASSERT by null-check in InsertedNodes::lastLeafInserted() as
m_lastNodeInserted can be null when this method is called. This ASSERT was
added in r161096 and we started hitting it in ClusterFuzz. This is also
consistent with InsertedNodes::pastLastLeaf() which already had a null
check.

R=adamk
BUG=338982
TEST=editing/execCommand/replaceSelectorCommand-crash.html

Review URL: https://codereview.chromium.org/149213006

git-svn-id: svn://svn.chromium.org/blink/trunk@166065 bbb929c8-8fbe-4397-9dbb-9b2b20218538