Avoid infinite loop in InsertListCommand::doApply

This patch changes loop condition in |while| loop to avoid infinite loop.
Original loop condition |!inSameParagraph(a,b)| isn't enough for
|startOfCurrentParagraph| is null case.

BUG=374138
TEST=LayoutTests/editing/execCommand/insert-list-infinite-loop.html

Review URL: https://codereview.chromium.org/296183003

git-svn-id: svn://svn.chromium.org/blink/trunk@174852 bbb929c8-8fbe-4397-9dbb-9b2b20218538