Fix #34783: smoke simulation crash when changing frame while preview rendering.

Added a mutex lock for smoke data access. The render was already working with a
copy of the volume data, so it's just a short lock to copy things and should not
block the UI much.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56273 954f8c5b-7b00-dc11-b283-0030488c597c