Fix parsing of un-numbered messages in series

Fix parsing of un-numbered messages in series

Say we are sent the following:

          - [PATCH 0/2] A sample series
            - [PATCH 1/2] test: Add some lorem ipsum
            - Random message with diff

We expect that:
 1) we parse normally without errors
 2) we get a series with a cover letter and a patch
 3) the random message is orphaned

What happens is that we get an integrity error, boiling down to:

(1048, "Column 'number' cannot be null")

This is caused because we believe that the random message belongs
to the series because of the headers, but because there are no
numbers in the Subject, we pass "None" into the number field of
SeriesPatch. That turns into a null, and rightly hits an integrity
error.

Fix this by requring that a message has a series _and_ a number
before we try to add it to the series.

Add a test to verify correctness.

Reported-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>