Fix autobox possibly writing to an outdated target

Since autobox is a macro, the target argument is not evaluated at the place of
the call but in the expanded code. That happens only after allocating, so the
GC may have already moved the target frame. Avoid this by forgoing the local
variable and accessing tc->cur_frame->caller again instead as that will have
been updated.

src/core/args.c

@@ -503,7 +503,7 @@ void MVM_args_set_result_int(MVMThreadContext *tc, MVMint64 result, MVMint32 fra
                 target->return_value->n64 = (MVMnum64)result;
                 break;
             case MVM_RETURN_OBJ:
-                autobox(tc, target, result, int_box_type, 0, set_int, target->return_value->o);
+                autobox(tc, target, result, int_box_type, 0, set_int, (frameless ? tc->cur_frame : tc->cur_frame->caller)->return_value->o);
                 break;
             default:
                 MVM_exception_throw_adhoc(tc, "Result return coercion from int NYI; expects type %u", target->return_type);
@@ -526,7 +526,7 @@ void MVM_args_set_result_num(MVMThreadContext *tc, MVMnum64 result, MVMint32 fra
                 target->return_value->i64 = (MVMint64)result;
                 break;
             case MVM_RETURN_OBJ:
-                autobox(tc, target, result, num_box_type, 0, set_num, target->return_value->o);
+                autobox(tc, target, result, num_box_type, 0, set_num, (frameless ? tc->cur_frame : tc->cur_frame->caller)->return_value->o);
                 break;
             default:
                 MVM_exception_throw_adhoc(tc, "Result return coercion from num NYI; expects type %u", target->return_type);
@@ -546,7 +546,7 @@ void MVM_args_set_result_str(MVMThreadContext *tc, MVMString *result, MVMint32 f
                 target->return_value->s = result;
                 break;
             case MVM_RETURN_OBJ:
-                autobox(tc, target, result, str_box_type, 1, set_str, target->return_value->o);
+                autobox(tc, target, result, str_box_type, 1, set_str, (frameless ? tc->cur_frame : tc->cur_frame->caller)->return_value->o);
                 break;
             default:
                 MVM_exception_throw_adhoc(tc, "Result return coercion from str NYI; expects type %u", target->return_type);