flush keepalives on empty return statements

Author: zsol

mypyc/irbuild/statement.py

@@ -70,6 +70,7 @@ def transform_return_stmt(builder: IRBuilder, stmt: ReturnStmt) -> None:
         retval = builder.accept(stmt.expr)
     else:
         retval = builder.builder.none()
+        builder.flush_keep_alives()
     retval = builder.coerce(retval, builder.ret_types[-1], stmt.line)
     builder.nonlocal_control[-1].gen_return(builder, retval, stmt.line)