#7 Fix null dereference with partial load on non-instruction

[easy_as_pi314]

I found this by accident when doing a code golf challenge (which I then used for a challenge and explained here 😜)

When a partial instruction load or store is done on a non-instruction (e.g. the end of the .text section), SPIM will attempt to read the instruction before checking if it is NULL, resulting in a segfault.

Proof of concept:

    .text
main:
    # also sh, lb, lh, lbu, and lhu
    sb  $a0, not_an_instruction
 not_an_instruction:
    # end of file

[James Larus]

Thanks! It is fixed in the next release.