[iOS] Bail out if the page proxy is invalid when WebPageProxy::processWillBecome...

[iOS] Bail out if the page proxy is invalid when WebPageProxy::processWillBecome{Foreground, Suspended}() are called
https://bugs.webkit.org/show_bug.cgi?id=151877

Reviewed by Darin Adler.

We cannot assume that a WebPageProxy is in a valid state when WebPageProxy::processWillBecome{Foreground, Suspended}()
are called because these callbacks may occur after the web process crashed and before a
WebPageProxy attaches to a new web process (and hence is considered in a valid state).

* UIProcess/WebPageProxy.cpp:
(WebKit::WebPageProxy::processWillBecomeSuspended): Early return if the page proxy is invalid.
(WebKit::WebPageProxy::processWillBecomeForeground): Ditto.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@193883 268f45cc-cd09-0410-ab3c-d52691b4dbfc