Silence uninitialized-value warnings in compareJsonbContainers().

Silence uninitialized-value warnings in compareJsonbContainers().

Because not every path through JsonbIteratorNext() sets val->type,
some compilers complain that compareJsonbContainers() is comparing
possibly-uninitialized values.  The paths that don't set it return
WJB_DONE, WJB_END_ARRAY, or WJB_END_OBJECT, so it's clear by
manual inspection that the "(ra == rb)" code path is safe, and
indeed we aren't seeing warnings about that.  But the (ra != rb)
case is much less obviously safe.  In Assert-enabled builds it
seems that the asserts rejecting WJB_END_ARRAY and WJB_END_OBJECT
persuade gcc 15.x not to warn, which makes little sense because
it's impossible to believe that the compiler can prove of its
own accord that ra/rb aren't WJB_DONE here.  (In fact they never
will be, so the code isn't wrong, but why is there no warning?)
Without Asserts, the appearance of warnings is quite unsurprising.

We discussed fixing this by converting those two Asserts into
pg_assume, but that seems not very satisfactory when it's so unclear
why the compiler is or isn't warning: the warning could easily
reappear with some other compiler version.  Let's fix it in a less
magical, more future-proof way by changing JsonbIteratorNext()
so that it always does set val->type.  The cost of that should be
pretty negligible, and it makes the function's API spec less squishy.

Reported-by: Erik Rijkers <er@xs4all.nl>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/988bf1bc-3f1f-99f3-bf98-222f1cd9dc5e@xs4all.nl
Discussion: https://postgr.es/m/0c623e8a204187b87b4736792398eaf1@postgrespro.ru
Backpatch-through: 13