Fix assignment in assert in any_type testcase (#580)

diff --git a/tests/any_type/decode_any.c b/tests/any_type/decode_any.c
index 6aa4b22..70e2508 100644
--- a/tests/any_type/decode_any.c
+++ b/tests/any_type/decode_any.c
@@ -37,7 +37,7 @@
         status = pb_decode(&stream, google_protobuf_Duration_fields, &duration);
         assert(status);
         assert(duration.seconds == 99999);
-        assert(duration.nanos = 100);
+        assert(duration.nanos == 100);
         return 0;
     }
     else