Fix NULL pointer dereference in remove_next_nth_ir()

There was a NULL pointer dereference issue in remove_next_nth_ir().
To address this, we now check for the existence of the next ir in the
loop. If there is no next ir, we terminate the loop early. This change
ensures that we do not attempt to remove non-existent ir.

src/emulate.c

@@ -729,6 +729,10 @@ static inline void remove_next_nth_ir(const riscv_t *rv,
 {
     for (uint8_t i = 0; i < n; i++) {
         rv_insn_t *next = ir->next;
+        if (!next) {
+            n = i;
+            break;
+        }
         ir->next = ir->next->next;
         mpool_free(rv->block_ir_mp, next);
     }