staging: gpib: Add lower bound check for secondary address

Commit 9dde4559e939 ("staging: gpib: Add GPIB common core driver")
from Sep 18, 2024 (linux-next), leads to the following Smatch static
checker warning:

drivers/staging/gpib/common/gpib_os.c:541 dvrsp()
warn: no lower bound on 'sad' rl='s32min-30'

The value -1 was introduced in user land to signify No secondary address
to the driver so that a lower bound check could be added.

This patch adds that check.

Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lore.kernel.org/linux-staging/[e-mail]/
Signed-off-by: Dave Penkler <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/gpib/common/gpib_os.c

···
 		return -1;
 	}
 
-	if (pad > MAX_GPIB_PRIMARY_ADDRESS || sad > MAX_GPIB_SECONDARY_ADDRESS) {
+	if (pad > MAX_GPIB_PRIMARY_ADDRESS || sad > MAX_GPIB_SECONDARY_ADDRESS || sad < -1) {
 		pr_err("gpib: bad address for serial poll");
 		return -1;
 	}