group: avoid divide by zero with empty group

Do not set empty group's stride to zero.

Alternatively we could add exceptions for empty groups, but it is
simpler if the arithmetic can work without exception.

Author: hzhou

src/mpi/group/grouputil.c

@@ -29,7 +29,12 @@ int MPIR_Group_init(void)
     MPIR_Group_builtin[0].size = 0;
     MPIR_Group_builtin[0].rank = MPI_UNDEFINED;
     MPIR_Group_builtin[0].session_ptr = NULL;
-    memset(&MPIR_Group_builtin[0].pmap, 0, sizeof(struct MPIR_Pmap));
+
+    MPIR_Group_builtin[0].pmap.size = 0;
+    MPIR_Group_builtin[0].pmap.use_map = false;
+    MPIR_Group_builtin[0].pmap.u.stride.offset = 0;
+    MPIR_Group_builtin[0].pmap.u.stride.stride = 1;
+    MPIR_Group_builtin[0].pmap.u.stride.blocksize = 1;
 
     return mpi_errno;
 }