Add forgotten setting of helper variables to zero after calculations

Integration results were incorrect, because the helper variables were
not reset after every iteration.

moose-core/biophysics/DifBuffer.cpp

@@ -381,7 +381,8 @@ void DifBuffer::vProcess( const Eref & e, ProcPtr p )
   prevFree_ = bFree_;
   prevBound_ = bBound_;
  
-
+  Af_ = 0;
+  Bf_= 0;
   /**
    * Send ion concentration to ion buffers. They will send back information on
    * the reaction (forward / backward rates ; free / bound buffer concentration)