gpib: agilent_82357a: don't check a NULL serial string

gpib: agilent_82357a: don't check a NULL serial string

The agilent_82357a driver uses the USB device serial string for device
matching but does not verify that the string exists before passing it
to strcmp().

Verify that the device has a serial number before accessing it to avoid
triggering a NULL-pointer dereference with devices that don't provide
a serial number (iSerialNumber = 0).

Similar to commit aa79f996eb41 ("i2c: cp2615: fix serial string
NULL-deref at probe").

Found by Claude:sonnet-4.5

Signed-off-by: Joe Simmons-Talbott <joest@redhat.com>
Acked-by: Dave Penkler <dpenkler@gmail.com>
Link: https://patch.msgid.link/20260326131256.1758014-1-joest@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>