Don't dereference null VisiblePosition in CompositeEditCommand::moveParagraphContentsToNewBlockIfNecessary

This patch changes CompositeEditCommand::moveParagraphContentsToNewBlockIfNecessary
to check whether VisiblePosition is null or not before dereferencing it.

BUG=371139
TEST=LayoutTests/editing/execCommand/justify-right-crash.html

Review URL: https://codereview.chromium.org/296973002

git-svn-id: svn://svn.chromium.org/blink/trunk@174828 bbb929c8-8fbe-4397-9dbb-9b2b20218538