Safari build crashes when "zooming into"/choosing a tab from tab view
https://bugs.webkit.org/show_bug.cgi?id=139934

Reviewed by Dan Bernstein.

* UIProcess/API/mac/WKView.mm:
(-[WKView _setIgnoresNonWheelEvents:]):
Trying to add a nil gesture recognizer raises an exception. Stop assuming that a
gesture recognizer is non-nil.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177754 268f45cc-cd09-0410-ab3c-d52691b4dbfc