tests: Fix undefined behaviour

Undefined behaviour in the test support code.

src/test_libFLAC/md5.c

@@ -167,8 +167,7 @@ static FLAC__byte target_digests [8][4][16] =
 static FLAC__bool test_md5_codec(void)
 {
 	FLAC__int32 arrays[MAX_CHANNEL_COUNT][MD5_SAMPLE_COUNT], *pointer[MAX_CHANNEL_COUNT], **signal;
-	FLAC__int32 seed = 0x12345679;
-	unsigned chan, byte_size;
+	unsigned chan, byte_size, seed = 0x12345679;
 
 	/* Set up signal data using a trival Linear Congruent PRNG. */
 	signal = &pointer[0];

src/test_libs_common/metadata_utils.c

@@ -338,7 +338,7 @@ FLAC__bool mutils__compare_block_data_picture(const FLAC__StreamMetadata_Picture
 		printf("FAILED, data_length mismatch, expected %u, got %u\n", block->data_length, blockcopy->data_length);
 		return false;
 	}
-	if(memcmp(blockcopy->data, block->data, block->data_length)) {
+	if(block->data_length > 0 && memcmp(blockcopy->data, block->data, block->data_length)) {
 		printf("FAILED, data mismatch\n");
 		return false;
 	}