Web Inspector: searching document with no documentElement results in 0x0 access.

Web Inspector: searching document with no documentElement results in 0x0 access.
https://bugs.webkit.org/show_bug.cgi?id=83483

Reviewed by Yury Semikhatsky.

Source/WebCore:

Added the null check.

Test: inspector/elements/dom-search-crash.html

* inspector/InspectorDOMAgent.cpp:
(WebCore::InspectorDOMAgent::performSearch):

LayoutTests:

* inspector/elements/dom-search-crash-expected.txt: Added.
* inspector/elements/dom-search-crash.html: Added.
* inspector/elements/resources/dom-search-crash-iframe.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113700 268f45cc-cd09-0410-ab3c-d52691b4dbfc