efficient and safe way to iterate on associative array?

[aki via Digitalmars-d-learn]

Is it okay to modify associative array while iterating it?
import std.stdio;

void main() {
        string[string] hash = [ "k1":"v1", "k2":"v2" ];
        auto r = hash.byKeyValue();
        while(!r.empty) {
                auto key = r.front.key;
                auto value = r.front.value;
                r.popFront();
                writefln("key=%s, value=%s", key, value);
                // may not modify 'hash' here ?
                hash = null;
        }
}

I guess probably it's not.
Then, my question is are there an efficient and safe way to 
iterate on an associative array even if there are possibility to 
be modified while iterating?
I'm writing interpreter and want to make my language to be safe; 
even malicious script cannot fall it in 'core dump' state. It is 
okay if it causes undefined behavior like throw or instant exit 
from loop, but not crash.

Thanks, Aki.