usb: Fix build with newer gcc

gcc 7 is pickier about our sources:

hw/usb/bus.c: In function ‘usb_port_location’:
hw/usb/bus.c:410:66: error: ‘%d’ directive output may be truncated writing between 1 and 11 bytes into a region of size between 0 and 15 [-Werror=format-truncation=]
         snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
                                                                  ^~
hw/usb/bus.c:410:9: note: ‘snprintf’ output between 3 and 28 bytes into a destination of size 16
         snprintf(downstream->path, sizeof(downstream->path), "%s.%d",
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  upstream->path, portnr);
                  ~~~~~~~~~~~~~~~~~~~~~~~

But we know that there are at most 5 levels of USB hubs, with at
most two digits per level; that plus the separating dots means we
use at most 15 bytes (including trailing NUL) of our 16-byte field.
Adding an assertion to show gcc that we checked for truncation is
enough to shut up the false-positive warning.

Inspired by an idea by Dr. David Alan Gilbert <dgilbert@redhat.com>.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170717151334.17954-1-eblake@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
