avoid invalid calls to memcpy(): when ndot == 0, pointers may be NULL

src/rcb.cpp

@@ -461,7 +461,7 @@ void RCB::compute(int dimension, int n, double **x, double *wt,
         largest = smaller;
         dim_select = dim;
         valuehalf_select = valuehalf;
-        memcpy(dotmark_select,dotmark,ndot*sizeof(int));
+        if (ndot > 0) memcpy(dotmark_select,dotmark,ndot*sizeof(int));
       }
     }
 
@@ -469,7 +469,7 @@ void RCB::compute(int dimension, int n, double **x, double *wt,
 
     dim = dim_select;
     valuehalf = valuehalf_select;
-    memcpy(dotmark,dotmark_select,ndot*sizeof(int));
+    if (ndot > 0) memcpy(dotmark,dotmark_select,ndot*sizeof(int));
 
     // found median
     // store cut info only if I am procmid