Bug fix when &vta returns empty absraction.

src/aig/gia/giaAbsGla.c

@@ -854,6 +854,7 @@ int Gia_GlaPerform( Gia_Man_t * pAig, Gia_ParVta_t * pPars )
         pPars->fDumpVabs    = nDumpOld;
         // create gate classes
         Vec_IntFreeP( &pAig->vGateClasses );
+        if ( pAig->vObjClasses )
             pAig->vGateClasses = Gia_VtaConvertToGla( pAig, pAig->vObjClasses );
         Vec_IntFreeP( &pAig->vObjClasses );
     }

src/aig/gia/giaAbsVta.c

@@ -1630,6 +1630,10 @@ finish:
     // analize the results
     if ( pCex == NULL )
     {
+        if ( Vec_PtrSize(p->vCores) == 0 )
+            Abc_Print( 1, "Abstraction is not produced because first frame is not solved.  " );
+        else
+        {
             assert( Vec_PtrSize(p->vCores) > 0 );
             if ( pAig->vObjClasses != NULL )
                 Abc_Print( 1, "Replacing the old abstraction by a new one.\n" );
@@ -1649,6 +1653,7 @@ finish:
             else
                 Abc_Print( 1, "SAT solver completed %d frames and produced an abstraction.  ", f );
         }
+    }
     else
     {
         ABC_FREE( p->pGia->pCexSeq );