Eliminate segfault on NSS and PAM responder startup.

If the data provider is not yet available when NSS and PAM start,
they will generate a segmentation fault when trying to configure
their automatic reconnection to the Data Provider. I've now added
code in sss_dp_init() to detect whether the dp_ctx is NULL and
return EIO.

server/responder/common/responder_common.c

@@ -554,6 +554,10 @@ int sss_process_init(TALLOC_CTX *mem_ctx,
         DEBUG(0, ("fatal error setting up backend connector\n"));
         return ret;
     }
+    else if (!rctx->dp_ctx) {
+        DEBUG(0, ("Data Provider is not yet available. Retrying.\n"));
+        return EIO;
+    }
 
     ret = sysdb_init(rctx, ev, cdb, NULL, &rctx->sysdb);
     if (ret != EOK) {