ASSERT when right clicking on SVG Image generating Share menu - can break Web Inspector

ASSERT when right clicking on SVG Image generating Share menu - can break Web Inspector
https://bugs.webkit.org/show_bug.cgi?id=150374

Patch by Joseph Pecoraro <pecoraro@apple.com> on 2015-10-20
Reviewed by Brian Burg.

When there is no absolute URL or downloadable media URL no system share
menu is created, but we were still appending the null context menu to
the list of context menu items.

* UIProcess/mac/WebContextMenuProxyMac.mm:
(WebKit::WebContextMenuProxyMac::shareMenuItem):
Bail early if the context menu is null.

(WebKit::WebContextMenuProxyMac::showContextMenu):
Only append the share context menu item if it is non-null.
This matches ContextMenuController.cpp in WebCore.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@191363 268f45cc-cd09-0410-ab3c-d52691b4dbfc