vm: Null pointer test when checking for StopIteration optimizations.

When generator raises exception, it is automatically terminated (by setting
its code_state.ip to 0), which interferes with this check.

Triggered in particular by CPython's test_pep380.py.

py/vm.c

@@ -1240,13 +1240,17 @@ exception_handler:
             code_state->ip -= 1;
             #endif
 
-            // check if it's a StopIteration within a for block
-            if (*code_state->ip == MP_BC_FOR_ITER && mp_obj_is_subclass_fast(mp_obj_get_type(nlr.ret_val), &mp_type_StopIteration)) {
-                const byte *ip = code_state->ip + 1;
-                DECODE_ULABEL; // the jump offset if iteration finishes; for labels are always forward
-                code_state->ip = ip + ulab; // jump to after for-block
-                code_state->sp -= 1; // pop the exhausted iterator
-                goto outer_dispatch_loop; // continue with dispatch loop
+            if (mp_obj_is_subclass_fast(mp_obj_get_type(nlr.ret_val), &mp_type_StopIteration)) {
+                if (code_state->ip) {
+                    // check if it's a StopIteration within a for block
+                    if (*code_state->ip == MP_BC_FOR_ITER) {
+                        const byte *ip = code_state->ip + 1;
+                        DECODE_ULABEL; // the jump offset if iteration finishes; for labels are always forward
+                        code_state->ip = ip + ulab; // jump to after for-block
+                        code_state->sp -= 1; // pop the exhausted iterator
+                        goto outer_dispatch_loop; // continue with dispatch loop
+                    }
+                }
             }
 
 #if MICROPY_STACKLESS