BUG: First occurence of alpha1.prevIter() causes crash (closes #911)

- in interIsoFoam/alphaEqn.H changed to alpha1.prevIter() to
  alpha1.storePrevIter().

applications/solvers/multiphase/interIsoFoam/alphaEqn.H

@@ -2,31 +2,32 @@
     if (pimple.nCorrPIMPLE() > 1)
     {
         // If nOuterCorrectors > 1 then for all but the first loop the advection
-        // of alpha is done using an average, 0.5*phi+0.5*phiNew where phi is 
-        // the flux at the beginning of the time step and phiNew is the flux 
-        // estimate at the end of the time step from the previous outer 
+        // of alpha is done using an average, 0.5*phi+0.5*phiNew where phi is
+        // the flux at the beginning of the time step and phiNew is the flux
+        // estimate at the end of the time step from the previous outer
         // iteration. Similarly we use 0.5*U + 0.5*UNew in later iterations.
         if (pimple.firstIter())
         {
-            // To recalculate the alpha1 update in subsequent iterations, we 
-            // must store its current value before overwriting with the new 
+            // To recalculate the alpha1 update in subsequent iterations, we
+            // must store its current value before overwriting with the new
             // value
-            alpha1.prevIter();
+            alpha1.storePrevIter();
             // Storing initial phi and U for use in later outer iterations.
             phi.storePrevIter();
             U.storePrevIter();
         }
         else
         {
-            // Resetting alpha1 to value before advection in first PIMPLE 
+            // Resetting alpha1 to value before advection in first PIMPLE
             // iteration.
             alpha1 = alpha1.prevIter();
+
             // Setting U and phi with which to advect interface.
             U = 0.5*U.prevIter() + 0.5*U;
             phi = 0.5*phi.prevIter() + 0.5*phi;
         }
     }
-    
+
     // Updating alpha1
     advector.advect();
     #include "rhofs.H"