spi: spidev_test: Fix buffer overflow in unescape()

Sometimes spidev_test crashes with:

    *** Error in `spidev_test': munmap_chunk(): invalid pointer: 0x00022020 ***
    Aborted

or just

    Segmentation fault

This is due to transfer_escaped_string() miscalculating the required
size of the buffer by one byte, causing a buffer overflow in unescape().

Drop the bogus "+ 1" in the strlen() parameter to fix this.

Note that unescape() never copies the zero-terminator of the source
string, so it writes at most as many bytes as the length of the source
string.

Fixes: 30061915be6e3a2c (spi: spidev_test: Added input buffer from the terminal)
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: <stable@vger.kernel.org> # v4.5+