FLAC__stream_encoder_finish: Sanity improvement

Previously this function would abort or crash if passed `NULL` for
the encoder. Now it just returns without crashing or aborting which
is far more sane behaviour (ie much like FLAC__stream_encoder_delete).

src/libFLAC/stream_encoder.c

@@ -1439,7 +1439,9 @@ FLAC_API FLAC__bool FLAC__stream_encoder_finish(FLAC__StreamEncoder *encoder)
 {
 	FLAC__bool error = false;
 
-	FLAC__ASSERT(0 != encoder);
+	if (encoder == NULL)
+		return false;
+
 	FLAC__ASSERT(0 != encoder->private_);
 	FLAC__ASSERT(0 != encoder->protected_);