Evaluating code in struct construction is a possible security issue

Such a code should not be allowed:
```rebol
>> s: make struct! [ a: [uint8] probe random 100 ]
25
>> mold s
== "make struct! [a: [uint8] 25]"
>> spec-of s
== [a: [uint8] probe random 100]
>> probe make s []
make struct! [a: [uint8] 25] ;<---- the spec is not used anymore
```