Bail out of WebView::paint when there's nothing to paint

        Fixes <http://webkit.org/b/38670> <rdar://problem/7947105> REGRESSION
        (r58067): Crash in WebView::paint when Web Inspector is docked and
        window is resized so small that WebView disappears

        When the WebView is 0-sized, ensureBackingStore() bails out without
        creating a bitmap, leaving m_backingStoreBitmap null. Before r58067,
        m_backingStoreBitmap was an HBITMAP, so we were happily passing along
        a null HBITMAP to various Windows APIs. These calls would fail but not
        crash. r58067 changed m_backingStoreBitmap to a RefCountedHBITMAP, and
        dereferencing a null RefCountedHBITMAP* of course crashes.

        Reviewed by Steve Falkenburg.

        * WebView.cpp:
        (WebView::paint): Bail if the rect to paint is empty.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@58900 268f45cc-cd09-0410-ab3c-d52691b4dbfc