Validate aom_tune_metric value.

Change-Id: I6d0f42ddec857921f5beb52b25925747e6412956

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 2ebdd3b..4a93191 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -381,6 +381,13 @@
 #if CONFIG_ANS && ANS_MAX_SYMBOLS
   RANGE_CHECK(extra_cfg, ans_window_size_log2, 8, 23);
 #endif
+
+#if CONFIG_DIST_8X8
+  RANGE_CHECK(extra_cfg, tuning, AOM_TUNE_PSNR, AOM_TUNE_DAALA_DIST);
+#else
+  RANGE_CHECK(extra_cfg, tuning, AOM_TUNE_PSNR, AOM_TUNE_SSIM);
+#endif
+
   return AOM_CODEC_OK;
 }