cmd_pxe: fix bootargs malloc size

Need a extra byte for the null termination.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
diff --git a/common/cmd_pxe.c b/common/cmd_pxe.c
index a1aa69a..9f1c14d 100644
--- a/common/cmd_pxe.c
+++ b/common/cmd_pxe.c
@@ -666,7 +666,7 @@
 		len += strlen(label->append);
 
 	if (len) {
-		bootargs = malloc(len);
+		bootargs = malloc(len + 1);
 		if (!bootargs)
 			return 1;
 		bootargs[0] ='\0';