Fix for bug reported by Dalai Felinto on IRC, crash in blenderplayer after r56055. The ntreeUpdateTree function uses G.main, which is not defined in blenderplayer (NULL). Using G.main is in itself an ugly hack required for the node verification. Just added a sanity check for now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56161 954f8c5b-7b00-dc11-b283-0030488c597c