LocalAccessValidator: fix validation of closures with no outer variables.

artiq/compiler/validators/local_access.py

@@ -99,6 +99,8 @@ class LocalAccessValidator:
 
                 if isinstance(insn, ir.Closure):
                     env = insn.environment()
+                    # Make sure this environment has any interesting variables.
+                    if env in block_state:
                         for var_name in block_state[env]:
                             if not block_state[env][var_name]:
                                 # A closure would capture this variable while it is not always