Assertion for logically decoding multi inserts into the catalog

My patch for using heap_multi_insert in the catalog [1] failed the logical
decoding part of test/recovery [2].

The assertion it failed on seems to not take multi inserts into the catalog
into consideration, while the main logic does.  This assertion hasn't tripped
since there are no multi inserts into the catalog, but if we introduce them it
will so I’m raising it in a separate thread as it is sort of unrelated from the
patch in question.

The attached patch fixes my test failure and makes sense to me, but this code
is far from my neck of the tree, so I’m really not sure this is the best way to
express the assertion.

cheers ./daniel

[1] https://commitfest.postgresql.org/23/2125/
[2] https://postgr.es/m/CA+hUKGLg1vFiXnkxjp_bea5+VP8D=vHRwSdvj7Rbikr_u4xFbg@mail.gmail.com