maps between GaloisFields

Nobody has claimed this yet.

When trying to create a map between two moderately large GaloisFields, we get the following behavior.

```
i14 : map(GF(101^12), GF(101^2))

11 10 9 8 7 6 5 4 3 2
o14 = map (GF 1126825030131969720661201, GF 10201, {30a + a - 28a + 39a + 18a - 26a + 28a + 48a + 9a - 9a + 10a + 32})

o14 : RingMap GF 1126825030131969720661201 <--- GF 10201

i15 : map(GF(101^14), GF(101^2))
stdio:15:1:(3): error: no method found for applying substitute to:
argument 1 : null (of class Nothing)
ZZ
argument 2 : ---[a] (of class PolynomialRing)
101
```

In particular, when the target field gets too big, it crashes. I tried messing around with the SizeLimit option, but that either doesn't help, or causes M2 itself to crash.

I haven't tried to figure out what is breaking yet.

Reproduce the two map commands shown in the issue, especially the GF(101^14) to GF(101^2) case, and inspect where the substitute operation receives null. Done means the larger-field map is created successfully without the reported error or a crash; the issue names no source files or tests to guide further investigation.

Written by the indexing model from the issue text.