Fix handling of throwing undef var in verify return

If we have an undefined variable and null is not accepted by the
return type, we want to throw just the undef var error.

In t

Fix handling of throwing undef var in verify return

If we have an undefined variable and null is not accepted by the
return type, we want to throw just the undef var error.

In this case this lead to an infinite loop, because we overwrite
the exception opline in SAVE_OPLINE and it does not get reset
when chaining into a previous exception. Add an assertiong to
catch this case earlier.

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