decoder: fixed assertion failure for decoder==NULL in decoder_read()

The assertion on dc.state in decoder_read() was too strict: when a
decoder tried to call decoder_read() from tag_dup(), the decoder state
was NONE.  Allow this special case.

src/decoder_api.c

@@ -100,7 +100,8 @@ size_t decoder_read(struct decoder *decoder,
 {
 	size_t nbytes;
 
-	assert(dc.state == DECODE_STATE_START ||
+	assert(decoder == NULL ||
+	       dc.state == DECODE_STATE_START ||
 	       dc.state == DECODE_STATE_DECODE);
 	assert(is != NULL);
 	assert(buffer != NULL);