08-defensive.md: move the last exercise to 'Extras' (#714)

The last exercise in `08-defensive.md` has a few issues:

a. it uses unclear language ("Fix" may refer to many things)
b. it uses functions not (currently) introduced in the lesson -- builtin `max` and `min`

Both issues can be fixed. Until then, we're moving this exercise to "Extras"

_episodes/08-defensive.md

@@ -538,27 +538,4 @@ This violates another important rule of programming:
 > {: .solution}
 {: .challenge}
 
-> ## Fixing and Testing
->
-> Fix `range_overlap`. Re-run `test_range_overlap` after each change you make.
->
-> > ## Solution
-> > ~~~
-> > def range_overlap(ranges):
-> >     '''Return common overlap among a set of [left, right] ranges.'''
-> >     if not ranges:
-> >         # ranges is None or an empty list
-> >         return None
-> >     max_left, min_right = ranges[0]
-> >     for (left, right) in ranges[1:]:
-> >         max_left = max(max_left, left)
-> >         min_right = min(min_right, right)
-> >     if max_left >= min_right:  # no overlap
-> >         return None
-> >     return (max_left, min_right)
-> > ~~~
-> > {: .language-python}
-> {: .solution}
-{: .challenge}
-
 {% include links.md %}

_extras/extra_exercises.md

@@ -49,3 +49,29 @@ or not (yet) added to the main lesson.
 > > list and then unpacked into `left` and `right`.
 > {: .solution}
 {: .challenge}
+
+> ## Fixing and Testing
+> From: "Defensive Programming"
+>
+> Fix `range_overlap`. Re-run `test_range_overlap` after each change you make.
+>
+> > ## Solution
+> > ~~~
+> > def range_overlap(ranges):
+> >     '''Return common overlap among a set of [left, right] ranges.'''
+> >     if not ranges:
+> >         # ranges is None or an empty list
+> >         return None
+> >     max_left, min_right = ranges[0]
+> >     for (left, right) in ranges[1:]:
+> >         max_left = max(max_left, left)
+> >         min_right = min(min_right, right)
+> >     if max_left >= min_right:  # no overlap
+> >         return None
+> >     return (max_left, min_right)
+> > ~~~
+> > {: .language-python}
+> {: .solution}
+{: .challenge}
+
+{% include links.md %}