LaTeX: longtable does not allow nested table

[jfbu]

Describe the bug

If Sphinx LaTeX writer encounters in a table cell a nested table, the parent table is not allowed to be rendered using "longtable" class.

writing... failed

Markup is unsupported in LaTeX:
index:: longtable does not support nesting a table.

How to Reproduce

$ yes "n" | sphinx-quickstart --dot _ --project "FOO" --author "JFB" -v 0 --release 0 --language en --suffix .rst --master index --makefile testtable
$ cd testtable
$ cat <<EOF >index.rst
TEST
====

test table in table
-------------------

.. list-table::
   :class: longtable

   * - source
     - source of auxiliary function invocation Values:

       +-----------+---+--+ 
       | INIT      | 0 |  | 
       +-----------+---+--+ 
       | RC        | 1 |  | 
       +-----------+---+--+ 
       | BUTTON    | 2 |  | 
       +-----------+---+--+ 
       | MAVLINK   | 3 |  | 
       +-----------+---+--+ 
       | MISSION   | 4 |  | 
       +-----------+---+--+ 
       | SCRIPTING | 5 |  | 
       +-----------+---+--+ 

EOF
$ make latex