bug fix: ProcForeach check condition needed a 'not'

When initializing the value of 'size' for the LoopCtx we need to
check that the 'size' has not yet been set. The bug only changed
the value of 'size' if it had already been set.

PhysicsTools/MVAComputer/src/ProcForeach.cc

@@ -104,7 +104,7 @@ void ProcForeach::eval(ValueIterator iter, unsigned int n) const
         auto& loopSize = iter.loopCtx().size();
 	while(iter) {
 		unsigned int size = iter.size();
-		if (loopSize)
+		if (!loopSize)
 			loopSize = size;
 
 		double value = iter[offset];