rhbz#1072607: (hopefully) fix crash in SvxRuler::MouseMove()

Not sure why mpParaItem is 0 here, add a check like in every other
SvxRuler method.

(regression from 47a77d7dbc427e51421e2df8d59695834cb74980)

Change-Id: Ic11a4d0c9b6ca217cd59d1d3f6789592996d2c1d
(cherry picked from commit 958616a323b7fe6b42cd642394cd90a9cd82e7f9)
Reviewed-on: https://gerrit.libreoffice.org/8492
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>