...ViewShellBase::GetEventMultiplexer call sequence, where

  OSL_ASSERT(mpImpl->mpEventMultiplexer.get()!=NULL);

would call on an already stale ViewShellBase::mpImpl if ~FromShellManager were
only called from within ViewShellBase::~Implementation instead of directly from
~ViewShellBase.

(Needed to unbreak <https://gerrit.libreoffice.org/#/c/18890> "boost->std" on
Mac OS X, where libc++ presumably nulls a std::unique_ptr early in ~unique_ptr.)

Change-Id: I179e0d12cc049f33b5724673b65f84d7f7c3d059