Add minor optimization

This commit was SVN r2806.

src/mpi/c/gather.c

@@ -95,6 +95,14 @@ int MPI_Gather(void *sendbuf, int sendcount, MPI_Datatype sendtype,
         }
     }
 
+    /* Can we optimize?  Everyone had to give the same send signature,
+       which means that everyone must have given a sendcount > 0 if
+       there's anything to send. */
+
+    if (sendcount == 0) {
+      return MPI_SUCCESS;
+    }
+
     /* Invoke the coll component to perform the back-end operation */
 	
     err = comm->c_coll.coll_gather(sendbuf, sendcount, sendtype, recvbuf,