Interface: Fix memory leak on opening ROMs, added function to manually free ROMs

[theCapypara]

As a side note, at a quick glance it seems that this also affects the Windows and Linux GUI frontends (didn't check Mac), since I couldn't find NDS_FreeROM(); being called there when opening a new ROM. The Windows GUI only seems to free the old ROM when explicitly told to close the ROM. However this is only from a quick glance at the code, I didn't actually do more investigation, but it might be worth to check.