hashtable: fix length calculation in hexport_r

The commit below incorrectly fixed hexport_r();
 >	size = totlen + 1;
One extra byte is necessary to NULL-terminate a whole buffer, "resp."

Fixes: f1b20acb4a03 ("hashtable: Fix length calculation in hexport_r")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/lib/hashtable.c b/lib/hashtable.c
index 1c48692..93028ed 100644
--- a/lib/hashtable.c
+++ b/lib/hashtable.c
@@ -662,7 +662,7 @@
 			return (-1);
 		}
 	} else {
-		size = totlen;
+		size = totlen + 1;
 	}
 
 	/* Check if the user provided a buffer */