ecma_uint32_to_utf8_string did not fill string as zero-terminated.

[lygstate]

The comment is ensure that

/**
 * ECMA-defined conversion of UInt32 to String (zero-terminated).
 *
 * See also:
 *          ECMA-262 v5, 9.8.1
 *
 * @return number of bytes copied to buffer
 */
lit_utf8_size_t
ecma_uint32_to_utf8_string (uint32_t value, /**< value to convert */
                            lit_utf8_byte_t *out_buffer_p, /**< buffer for string */
                            lit_utf8_size_t buffer_size) /**< size of buffer */