libio: Fix buffer overrun in tst-ftell-active-handler

On 'do_ftell_test' the code:

365           if (test_modes[i].fd_mode != O_WRONLY)
366             {
367               char tmpbuf[data_len];
368
369               rewind (fp);
370
371               while (fgets_func (tmpbuf, sizeof (tmpbuf), fp) && !feof (fp));

The 'data_len' is calculated with wsclen and allocated as 'char'.  The
subsequent fgetws will then try to write at most 'data_len' wchar_t
in a buffer with just data_len 'char'.  This patch fixes it by
allocating the tmpbuf using 'wchar_t' * data_len bytes.