git bisect showed that a821ef5 already fixed this indirectly,
by showing the following error message:
  Local symbols can't be referenced in 'Dim Byref As UDT1 UDT2.ru1 = u1'
now since 3cc22e2 another error is shown:
  Expected constant in 'Dim Byref As UDT1 UDT2.ru1 = u1'

The CONST code path in hFlushExprStatic() used the symbol type as-is,
instead of converting to pointer type for references. Thus the type
looked different and it attempted the astNewCONV() which could easily
fail (because the init expression is a pointer, while the symbol could
be anything, e.g. struct/string).

I can't think of a test case for this though, CONSTness should have been
checked before already, it shouldn't matter here when emitting.
Also the full type is only used for an astNewCONV() which allows casting
away CONSTness etc. anyways.