HID: core: detect and skip invalid inputs to snto32()

Prevent invalid (0, 0) inputs to hid-core's snto32() function.

Maybe it is just the dummy device here that is causing this, but
there are hundreds of calls to snto32(0, 0). Having n (bits count)
of 0 is causing the current UBSAN trap with a shift value of
0xffffffff (-1, or n - 1 in this function).

Either of the value to shift being 0 or the bits count being 0 can be
handled by just returning 0 to the caller, avoiding the following
complex shift + OR operations:

	return value & (1 << (n - 1)) ? value | (~0U << n) : value;

Fixes: dde5845a ("[PATCH] Generic HID layer - code split")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: syzbot+1e911ad71dd4ea72e04a@syzkaller.appspotmail.com
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/hid-core.c

@@ -1307,6 +1307,9 @@ EXPORT_SYMBOL_GPL(hid_open_report);
 
 static s32 snto32(__u32 value, unsigned n)
 {
+	if (!value || !n)
+		return 0;
+
 	switch (n) {
 	case 8:  return ((__s8)value);
 	case 16: return ((__s16)value);