pci-assign: Fix potential read beyond buffer on -EBUSY

readlink() doesn't write a terminating null byte.
assign_failed_examine() passes the unterminated string to strrchr().
Oops.  Terminate it.

Spotted by Coverity.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAlex Williamson <alex.williamson@redhat.com>

hw/i386/kvm/pci-assign.c

@@ -743,6 +743,7 @@ static void assign_failed_examine(AssignedDevice *dev)
         goto fail;
     }
 
+    driver[r] = 0;
     ns = strrchr(driver, '/');
     if (!ns) {
         goto fail;