CRASH: when making a struct with another struct field and initial value is missing

this is ok:
```rebol
y: make struct! [ c [struct! [a [uint8!]]] ]
```
this is also ok:
```rebol
x: make struct! [ a [uint8!] ]
y: make struct! [ c: [struct! [a [uint8!]]] x ]
```
but this is not:
```rebol
y: make struct! [ c: [struct! [a [uint8!]]] ] ;<--- CRASH (as there is expected the value)
```