regression: stack-overflow when validating recursive data-structures in initialization code

[mhasel]

regressed commit: 41968d3

When compiling the following cyclic data-structures:

FUNCTION_BLOCK bar
VAR
    f : foo;
    ff AT f : foo;
END_VAR
END_FUNCTION_BLOCK

FUNCTION_BLOCK foo
VAR
    f : bar;
END_VAR
END_FUNCTION_BLOCK

the cycle will be reported during validation, but when the generated init-functions are validated afterwards, validate_assignment_type_sizes seems to be called ad infinum.