fuzz: add a 'to_string' function to libfuzz

We want to use it outside of fuzz-amount

Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

tests/fuzz/libfuzz.c

@@ -31,3 +31,13 @@ const uint8_t **get_chunks(const void *ctx, const uint8_t *data,
 
 	return chunks;
 }
+
+char *to_string(const tal_t *ctx, const u8 *data, size_t data_size)
+{
+	char *string = tal_arr(ctx, char, data_size);
+
+	for (size_t i = 0; i < data_size; i++)
+		string[i] = (char) data[i] % (CHAR_MAX + 1);
+
+	return string;
+}