vp8e_encode: check validate_config return

Change-Id: I56fad85cae4537f9d68febf9b9264c3e1cf82ce1
diff --git a/vp8/vp8_cx_iface.c b/vp8/vp8_cx_iface.c
index fe88cd4..960c2f4 100644
--- a/vp8/vp8_cx_iface.c
+++ b/vp8/vp8_cx_iface.c
@@ -879,7 +879,8 @@
     }
     ctx->control_frame_flags = 0;
 
-    res = set_reference_and_update(ctx, flags);
+    if (!res)
+        res = set_reference_and_update(ctx, flags);
 
     /* Handle fixed keyframe intervals */
     if (ctx->cfg.kf_mode == VPX_KF_AUTO