IOMUX: Fix buffer overflow in iomux_replace_device()

Use of strcat() against an uninitialized buffer would lead
to buffer overflow. This patch fixes it.

Fixes: 694cd561 ("IOMUX: Introduce iomux_replace_device()")
Signed-off-by: Yuichiro Goto <goto@k-tech.co.jp>
Cc: Peter Robinson <pbrobinson@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>

common/iomux.c

@@ -158,8 +158,12 @@ int iomux_replace_device(const int console, const char *old, const char *new)
 			return -ENOMEM;
 		}
 
-		strcat(tmp, ",");
-		strcat(tmp, name);
+		if (arg) {
+			strcat(tmp, ",");
+			strcat(tmp, name);
+		}
+		else
+			strcpy(tmp, name);
 
 		arg = tmp;
 		size = strlen(tmp) + 1;