Fixes bug 483

prevent crash when surface mobility (surfmob) has been
selected in 2D simulation.
The patch is provided by Brian Taylor (see
https://sourceforge.net/p/ngspice/mailman/ngspice-devel/thread/e6e921a3-677c-f513-7410-df8568c4bdad%40comcast.net/#msg37318450
)

src/ciderlib/twod/twocont.c

@@ -253,7 +253,7 @@ void
 	      pNode->fPPsiOx   = spGetElement( matrix, pEqn, psiEqnOxP );
 	    }
 	  } else { /* Horizontal Slice */
-	    if ( nIndex <= 1 ) { /* Top Side */
+	    if ( nIndex == 0 || nIndex == 3 ) { /* Left (Top?) Side : bug 483 */
 	      pNode->fNPsiIn   = spGetElement( matrix, nEqn, psiEqnInM );
 	      pNode->fNPsiInP1 = spGetElement( matrix, nEqn, psiEqnInP );
 	      pNode->fNPsiOx   = spGetElement( matrix, nEqn, psiEqnOxM );

src/ciderlib/twod/twoncont.c

@@ -211,7 +211,7 @@ void
 	      pNode->fNPsiOx   = spGetElement( matrix, nEqn, psiEqnOxP );
 	    }
 	  } else { /* Horizontal Slice */
-	    if ( nIndex <= 1 ) { /* Top Side */
+	    if ( nIndex == 0 || nIndex == 3 ) { /* Left (Top?) Side : bug 483 */
 	      pNode->fNPsiIn   = spGetElement( matrix, nEqn, psiEqnInM );
 	      pNode->fNPsiInP1 = spGetElement( matrix, nEqn, psiEqnInP );
 	      pNode->fNPsiOx   = spGetElement( matrix, nEqn, psiEqnOxM );

src/ciderlib/twod/twopcont.c

@@ -210,7 +210,7 @@ void
 	      pNode->fPPsiOx   = spGetElement( matrix, pEqn, psiEqnOxP );
 	    }
 	  } else { /* Horizontal Slice */
-	    if ( nIndex <= 1 ) { /* Top Side */
+	    if ( nIndex == 0 || nIndex == 3 ) { /* Left (Top?) Side : bug 483 */
 	      pNode->fPPsiIn   = spGetElement( matrix, pEqn, psiEqnInM );
 	      pNode->fPPsiInP1 = spGetElement( matrix, pEqn, psiEqnInP );
 	      pNode->fPPsiOx   = spGetElement( matrix, pEqn, psiEqnOxM );