Clamp loop_start/loop_length in delPart to ensure setupPingPingLoop doesn't occasionally read/write past end of sample buffer

[prayerie]

Rare edge case I found with asie's PC port and then confirmed manually in the NDS version.
Occasionally when deleting parts of audio data with a ping-pong looping sample, loop_start / loop_length can exceed the end of the sample by a couple of bytes (please do confirm in case I'm incorrect though). Adding these seemed to resolve the issue (there is still a tiny buffer overread in setupPingPongLoop, but it's unrelated and presumably benign).

[prayerie]

Since opening this pull request there was somewhere else in sample.cpp that also needed these values clamping, so I'll make that change in a bit

[prayerie]

Actually I tested around a bit and I must have misremembered, this is the only clamping necessary; the other thing was changing memcpy to memmove