svc_encoder_rtc.c: Treat decoder->err as private

Treat decoder->err as a private data member. Do not manipulate it
directly.

As a result, call die() instead of die_codec() because the mismatch is
not caused by the failure of an aom_codec_* function.

Bug: aomedia:3404
Change-Id: If2e70ec54bfe9a15679a159fd607a47910e8646b
diff --git a/examples/svc_encoder_rtc.c b/examples/svc_encoder_rtc.c
index 485d5a8..eb874de 100644
--- a/examples/svc_encoder_rtc.c
+++ b/examples/svc_encoder_rtc.c
@@ -1172,7 +1172,6 @@
 #else
     aom_find_mismatch(&enc_img, &dec_img, y, u, v);
 #endif
-    decoder->err = 1;
     fprintf(stderr,
             "Encode/decode mismatch on frame %d at"
             " Y[%d, %d] {%d/%d},"
@@ -1670,7 +1669,7 @@
                     mismatch_seen);
             fclose(stats_file);
 #endif
-            die_codec(&decoder, "Mismatch seen");
+            die("Mismatch seen");
           }
         }
       }