Avoid allocating a new DocumentFragment when tryFastParsingHTMLFragment() fails.
Pull request: https://github.com/WebKit/WebKit/pull/10249
Committed 260424@main (5824bab256ea): <https://commits.webkit.org/260424@main> Reviewed commits have been landed. Closing PR #10249 and removing active labels.
<rdar://problem/105581351>