stlink: swo: use completely the available buffer

stlink: swo: use completely the available buffer

The buffer passed to stlink_usb_trace_read() is allocated of size
*size and does not need to be zero-terminated. There is no reason
to not fill its last byte.

When checking the bytes available on swo, limit the retrieved byte
length to *size.

Change-Id: Iade0f8963118695931f13a8a3f1ab204911236b6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Adrian Negreanu <adrian.negreanu@nxp.com>
Reviewed-on: http://openocd.zylin.com/6061
Tested-by: jenkins
Reviewed-by: Adrian M Negreanu <adrian.negreanu@nxp.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>