mcexec: fix strncat bounding

strncat must not look at the appendee's length, but at how much
is left where we're appending.
This API is stupid anyway, where is strlcat when we need it...

Change-Id: Icdf418083146420a06f8ba5ffdf882982610d39b

executer/user/mcexec.c

@@ -3311,8 +3311,10 @@ int main_loop(struct thread_data_s *my_thread)
 					                  0);
 					break;
 				}
-				strncat(pathbuf, "/", 1);
-				strncat(pathbuf, tmpbuf, strlen(tmpbuf) + 1);
+				strncat(pathbuf, "/",
+					sizeof(pathbuf) - strlen(pathbuf) - 1);
+				strncat(pathbuf, tmpbuf,
+					sizeof(pathbuf) - strlen(pathbuf) - 1);
 			}
 			else {
 			}