Check nullable value before using it in ReplaceSelectionCommand::removeRedundantStylesAndKeepStyleSpanInline

This patch changes removeRedundantStylesAndKeepStyleSpanInline() to check
nullable value |element->parentNode()| before using it. L529, if-statement
just after this change, checks |element->parentNode()| too.

BUG=369422
TEST=LayoutTests/editing/execCommand/insert-html-to-document-element-crash.html

Review URL: https://codereview.chromium.org/300143012

git-svn-id: svn://svn.chromium.org/blink/trunk@175019 bbb929c8-8fbe-4397-9dbb-9b2b20218538