Fix buffer overflow

Add check for bounds of sindex[] and rindex[].

Signed-off-by: Mikhail Kurnosov <mkurnosov@gmail.com>

ompi/mca/coll/spacc/coll_spacc_allreduce.c

@@ -276,11 +276,13 @@ int mca_coll_spacc_allreduce_intra_redscat_allgather(
                            rcount[step], dtype);
 
             /* Move the current window to the received message */
+            if (step + 1 < nsteps) {
                 rindex[step + 1] = rindex[step];
                 sindex[step + 1] = rindex[step];
                 wsize = rcount[step];
                 step++;
             }
+        }
         /*
          * Assertion: each process has 1 / p' of the total reduction result:
          * rcount[nsteps - 1] elements in the rbuf[rindex[nsteps - 1], ...].

ompi/mca/coll/spacc/coll_spacc_reduce.c

@@ -291,12 +291,14 @@ int mca_coll_spacc_reduce_intra_redscat_gather(
                            rcount[step], dtype);
 
             /* Move the current window to the received message */
+            if (step + 1 < nsteps) {
                 rindex[step + 1] = rindex[step];
                 sindex[step + 1] = rindex[step];
                 wsize = rcount[step];
                 step++;
             }
         }
+    }
     /*
      * Assertion: each process has 1 / p' of the total reduction result:
      * rcount[nsteps - 1] elements in the rbuf[rindex[nsteps - 1], ...].