Missing null check

As Kendy found out, "In Writer: Enable the drawing functions [view -> toolbars
-> drawing], choose 'Line Callout 1' among the presets Callouts [to the left
from the stars], and attempt to click and drag in the document to insert it"
triggers assert in SvRef<T>::operator* introduced in
160ae9889e4d16217a7cca7d930f776f5a645ec8 "Catch illegal null pointer
dereferences early."

Given how most places in sfx2/source/doc/sfxbasemodel.cxx already check for null
m_pObjectShell, looks like this had just been missing here, and the
SfxDocInfoListener_Impl's modified has probably never been called, so it went
unnoticed that its m_rShell was a "null reference."

Change-Id: I6b6dad831938d5fa89340895b98797686b60c5cc