Guard against out-of-bounds jumps in spesh arg guard program.

This seems to be what sometimes happens on arm builds for our debian
package, and hitting this oops should be better than running into an
endless do-nothing loop.

Author: timo

src/spesh/arg_guard.c

@@ -510,8 +510,13 @@ MVMint32 MVM_spesh_arg_guard_run(MVMThreadContext *tc, MVMSpeshArgGuard *ag,
                 break;
             case MVM_SPESH_GUARD_OP_RESULT:
                 return agn->result;
+            default:
+                MVM_oops(tc, "Unknown spesh arg guard opcode %d reached", agn->op);
         }
-    } while (current_node != 0);
+    } while (current_node != 0 && current_node < ag->num_nodes);
+    if (current_node >= ag->num_nodes) {
+        MVM_oops(tc, "Spesh arg guard program jumped out of bounds: index %u is above %u", current_node, ag->num_nodes);
+    }
     return -1;
 }