Currently ver_ptr is being null checked twice, once before calling
usb_string and once afterwards.  The second null check is redundant
and can be removed, remove it.

Detected by CoverityScan, CID#1477308 ("Logically dead code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>