Fixing crash in doApplyForSingleParagraph().

enclosingListChild() method can return 0 but no null check is implemented
at its calling location.
Adding a null check for the same.

Note that the corresponding bug mentions r163661 as being the cause for
this crash. However the crash persists even after reverting r163661.

BUG=345373
Test=editing/execCommand/crash-inserting-list.html

Review URL: https://codereview.chromium.org/180003007

git-svn-id: svn://svn.chromium.org/blink/trunk@167991 bbb929c8-8fbe-4397-9dbb-9b2b20218538