btl/ofi: Fix valgrind complaints on uninitialized pointer use

It doesn't seem like the BTL was using uninitialized pointer. But simply
setting the rcache pointer to NULL after destroying it makes the valgrind
errors go away.

Fixes Issue #6345

Signed-off-by: Aravind Gopalakrishnan <Aravind.Gopalakrishnan@intel.com>
(cherry picked from commit 786e686d4347655b574e609c65626c8323bb49b2)
Signed-off-by: Brian Barrett <bbarrett@amazon.com>

opal/mca/btl/ofi/btl_ofi_module.c

@@ -313,6 +313,7 @@ int mca_btl_ofi_finalize (mca_btl_base_module_t* btl)
 
     if (ofi_btl->rcache) {
         mca_rcache_base_module_destroy (ofi_btl->rcache);
+        ofi_btl->rcache = NULL;
     }
 
     free (btl);