Fix buffer overflow in RSP DMA - SECURITY CONCERN

[bkimmett]

Recently, ParaLLeL launcher has fixed a bug in its version of its RSP code that allowed N64 code to potentially escape the emulator and perform ACE on the user's computer (see https://www.twitch.tv/videos/2147408942?t=12257s for an example of this escape in action).

The fix is here: https://gitlab.com/parallel-launcher/parallel-n64/-/commit/fa9e2e08b0c6cde117cb05cc071cdb74a9e90ad2#06a9a5c035040370b6d9d811526d9586179b3176

The equivalent code in Delta's version of the Mupen core, https://github.com/rileytestut/mupen64plus-core/blob/aa9903b5446a9b50b8f5a31f927ca98e5a33a230/src/device/rcp/rsp/rsp_core.c#L58 and https://github.com/rileytestut/mupen64plus-core/blob/aa9903b5446a9b50b8f5a31f927ca98e5a33a230/src/device/rcp/rsp/rsp_core.c#L84, appears to also be vulnerable to this.

I strongly recommend fixing this as soon as possible.