Prevent segfault when the USB product string cannot be retrieved

Prevent segfault when the USB product string cannot be retrieved

In the CMSIS-DAP driver, if nothing has been specified by the user, an
attempt is made to find the first device with the (mandatory)
substring "CMSIS-DAP" in any USB device's product string.  However,
while (usually) all devices can be traversed, devices the user does
not have permission for cannot be read the product string from,
resulting in a NULL pointer.  Trying to find the substring "CMSIS-DAP"
causes a segementation fault then.

This has also been filed as Trac bug #67:

https://sourceforge.net/apps/trac/openocd/ticket/67

Change-Id: Idfc9f072e34152e9af99fe1c8ec88c99dea4624c
Signed-off-by: Jörg Wunsch <openocd@uriah.heep.sax.de>
Reviewed-on: http://openocd.zylin.com/2044
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>