JIT: fix possible block map crash with ISI blocks

If a block ISIs at the start (zero instructions) the logic here can fail and
cause a crash.
Yeah, the only way the value is actually used is to sort the map and for a call to std::map::lower_bound.  Offsetting it by 3 should be fine.

On a side note, it would be a lot cleaner conceptually to ban zero-length blocks, e.g. https://github.com/magumagu/dolphin/compare/simplify-isi (WIP, probably won't have time to finish for a couple days).