Pickling fails for some residue fields

[nthiery]

Caught with #7921; please write patch on top of it to avoid conflicts.

sage: K.<a> = NumberField(x^3-17)
sage: P = K.ideal(29).factor()[0][0]
sage: k = K.residue_field(P) # indirect doctest
sage: F = ZZ.residue_field(17)  # indirect doctest
sage: loads(dumps(k))
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "sage_object.pyx", line 740, in sage.structure.sage_object.dumps (sage/structure/sage_object.c:8021)
  File "ring.pyx", line 2424, in sage.rings.ring.FiniteField.__reduce__ (sage/rings/ring.c:12853)
TypeError: 'NoneType' object is unsubscriptable

sage: loads(dumps(k.an_element()))
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "sage_object.pyx", line 740, in sage.structure.sage_object.dumps (sage/structure/sage_object.c:8021)
  File "ring.pyx", line 2424, in sage.rings.ring.FiniteField.__reduce__ (sage/rings/ring.c:12853)
TypeError: 'NoneType' object is unsubscriptable

sage: loads(dumps(F))
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "sage_object.pyx", line 740, in sage.structure.sage_object.dumps (sage/structure/sage_object.c:8021)
  File "ring.pyx", line 2424, in sage.rings.ring.FiniteField.__reduce__ (sage/rings/ring.c:12853)
TypeError: 'NoneType' object is unsubscriptable

sage: loads(dumps(F.an_element()))
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
  File "sage_object.pyx", line 740, in sage.structure.sage_object.dumps (sage/structure/sage_object.c:8021)
  File "ring.pyx", line 2424, in sage.rings.ring.FiniteField.__reduce__ (sage/rings/ring.c:12853)
TypeError: 'NoneType' object is unsubscriptable

Component: number fields

Reviewer: David Roe

Issue created by migration from https://trac.sagemath.org/ticket/7929

[nthiery]

Description changed:

--- 
+++ 
@@ -1,4 +1,4 @@
-Catched with #7921:
+Caught with #7921; please write patch on top of it to avoid conflicts.
 
 ```
 sage: K.<a> = NumberField(x^3-17)

[roed314]

comment:2

This works in 4.7.2.

[jdemeyer]

Reviewer: David Roe