objtool, Input: cyapa - Remove undefined behavior in cyapa_update_fw_store()

In cyapa_update_fw_store(), if 'count' is zero, the write to
fw_name[count-1] underflows the array.

Presumably that's not possible in normal operation, as its caller
sysfs_kf_write() already checks for zero.  Regardless it's a good idea
to check for the error explicitly and avoid undefined behavior.

Fixes the following warning with an UBSAN kernel:

  vmlinux.o: warning: objtool: .text.cyapa_update_fw_store: unexpected end of section

Reported-by: kernel test robot <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Dmitry Torokhov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/73ae0bb3c656735890d914b74c9d6bb40c25d3cd.1742852847.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/

Author: jpoimboe

drivers/input/mouse/cyapa.c

@@ -1080,8 +1080,8 @@ static ssize_t cyapa_update_fw_store(struct device *dev,
 	char fw_name[NAME_MAX];
 	int ret, error;
 
-	if (count >= NAME_MAX) {
-		dev_err(dev, "File name too long\n");
+	if (!count || count >= NAME_MAX) {
+		dev_err(dev, "Bad file name size\n");
 		return -EINVAL;
 	}