Drivers: misc: fix out-of-bounds access in function param_set_kgdbts_var

There is an out-of-bounds access to "config[len - 1]" array when the
variable "len" is zero.

See commit dada6a43 ("kgdboc: fix KASAN global-out-of-bounds bug
in param_set_kgdboc_var()") for details.
Signed-off-by: Young Xiao <YangX92@hotmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/kgdbts.c

@@ -1135,7 +1135,7 @@ static void kgdbts_put_char(u8 chr)
 static int param_set_kgdbts_var(const char *kmessage,
 				const struct kernel_param *kp)
 {
-	int len = strlen(kmessage);
+	size_t len = strlen(kmessage);
 
 	if (len >= MAX_CONFIG_LEN) {
 		printk(KERN_ERR "kgdbts: config string too long\n");
@@ -1155,7 +1155,7 @@ static int param_set_kgdbts_var(const char *kmessage,
 
 	strcpy(config, kmessage);
 	/* Chop out \n char as a result of echo */
-	if (config[len - 1] == '\n')
+	if (len && config[len - 1] == '\n')
 		config[len - 1] = '\0';
 
 	/* Go and configure with the new params. */