Undo last step just goes back one instruction instead of "back jumping" to correct instruction.

Using the following sample program 

```riscv
# Exemple d'utilisation de jr
li a0, 421
auipc s0, 0		# s0 = pc
addi s0, s0, 16
jr s0			# Branche à l'adresse s0
li a0, 42
li a7, 1 		# PrintInt
ecall
li a7, 10
ecall
#stdout:421
#only:rars
```

When using the debug step process, going back after the jr will go back on instruction instead of going back to the jr.