Added skipping &iso when there is only one PO.

src/aig/gia/giaIso.c

@@ -977,6 +977,8 @@ Gia_Man_t * Gia_ManIsoReduce( Gia_Man_t * p, Vec_Ptr_t ** pvPosEquivs, int fVerb
 
     // create preliminary equivalences
     vEquivs = Gia_IsoDeriveEquivPos( p, 1, fVeryVerbose );
+    if ( vEquivs == NULL )
+        return NULL;
 //    printf( "Reduced %d outputs to %d outputs.  ", Gia_ManPoNum(p), Vec_PtrSize(vEquivs) );
 //    Abc_PrintTime( 1, "Time", clock() - clk );
 

src/base/abci/abc.c

@@ -27966,6 +27966,11 @@ int Abc_CommandAbc9Iso( Abc_Frame_t * pAbc, int argc, char ** argv )
         return 1;
     }
     pAig = Gia_ManIsoReduce( pAbc->pGia, &vPosEquivs, fVerbose );
+    if ( pAig == NULL )
+    {
+        Abc_Print( -1, "Abc_CommandAbc9Iso(): Transformation has failed.\n" );
+        return 1;
+    }
     // update the internal storage of PO equivalences
     Abc_FrameReplacePoEquivs( pAbc, &vPosEquivs );
     // update the AIG