DAOS-6686 cart: Fix coverity defect

[dmiter]

Fix possible NULL pointer dereference on error pass handling.

    	22. assign_zero: Assigning: rpc_priv->crp_hg_hdl = NULL.
 884                        rpc_priv->crp_hg_hdl = NULL;
 885                        RPC_ERROR(rpc_priv,
 886                                  "HG_Reset failed, hg_ret: %d\n",
 887                                  hg_ret);
 888                        rc = -DER_HG;
 889                }
 890        }
 891
 892        if (crt_gdata.cg_sep_mode == true) {
    	
CID 290781 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
28. var_deref_model: Passing null pointer rpc_priv->crp_hg_hdl to HG_Set_target_id, which dereferences it. [hide details]
 893                hg_ret = HG_Set_target_id(rpc_priv->crp_hg_hdl,
/install/prereq/release/mercury/include/mercury.h
1110HG_Set_target_id(hg_handle_t handle, hg_uint8_t id)
1111{
    	1. deref_parm: Directly dereferencing parameter handle.
1112    handle->info.context_id = id;
1113
1114    return HG_Core_set_target_id(handle->core_handle, id);
1115}

[daosbuild1]

LGTM. No errors found by checkpatch.

[jolivier23]

Please add gatekeeper back after testing is complete