mu: fix strncpy usage

Ensure the resulting strings are \0-terminated.

mu/mu-cmd-find.c

@@ -344,7 +344,8 @@ field_string_list (MuMsg *msg, MuMsgFieldId mfid)
 
 	str = mu_str_from_list (lst, ',');
 	if (str) {
-		strncpy (buf, str, sizeof(buf));
+		strncpy (buf, str, sizeof(buf)-1);
+		buf[sizeof(buf)-1]='\0';
 		g_free (str);
 		return buf;
 	}

mu/mu-cmd-server.c

@@ -1406,7 +1406,8 @@ get_path_from_docid (MuStore *store, unsigned docid, GError **err)
 		return NULL;
 	}
 
-	strncpy (path, msgpath, sizeof(path));
+	strncpy (path, msgpath, sizeof(path) - 1);
+	path[sizeof(path)-1] = '\0';
 
 	mu_msg_unref (msg);
 	return path;