Check nullable value before using it in IndentOutdentCommand::tryIndentingAsListItem

This patch change IndentOutdentCommand::tryIndentingAsListItem to check nullable
value returned by encloseBlock() before using it.

BUG=376721
TEST=LayoutTests/editing/execCommand/indent-inline-box-crash.html

Review URL: https://codereview.chromium.org/297203004

git-svn-id: svn://svn.chromium.org/blink/trunk@174952 bbb929c8-8fbe-4397-9dbb-9b2b20218538