Reject array insert(pos, first, last) iterators not pointing into an array

The array-range insert() overload checked that pos fits the current
value and that first/last share the same owning value, but never
verified that value is itself an array. Passing iterators from an
object, a primitive, or null handed value-initialized (singular)
std::vector iterators straight to array_t::insert(), which is
undefined behavior. Add the missing is_array() check, mirroring the
equivalent check already present in the object-range insert()
overload.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/insert.md

@@ -88,6 +88,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
       do not belong to the same JSON value; example: `"iterators do not fit"`
     - Throws [`invalid_iterator.211`](../../home/exceptions.md#jsonexceptioninvalid_iterator211) if `first` or `last`
       are iterators into container for which insert is called; example: `"passed iterators may not belong to container"`
+    - Throws [`invalid_iterator.202`](../../home/exceptions.md#jsonexceptioninvalid_iterator202) if `first` or `last`
+      do not point to an array; example: `"iterators first and last must point to arrays"`
 4. The function can throw the following exceptions:
     - Throws [`type_error.309`](../../home/exceptions.md#jsonexceptiontype_error309) if called on JSON values other than
       arrays; example: `"cannot use insert() with string"`