Suppress compiler warnings from commit ee895a655.

Suppress compiler warnings from commit ee895a655.

For obscure reasons, some buildfarm members are now generating
complaints about plpgsql_call_handler's "retval" variable possibly
being used uninitialized.  It seems no less safe than it was before
that commit, but these complaints are (mostly?) new.  I trust that
initializing the variable where it's declared will be enough to
shut that up.

I also notice that some compilers are warning about setjmp clobber
of the same variable, which is maybe a bit more defensible.  Mark
it volatile to silence that.

Also, rearrange the logic to give procedure_resowner a single
point of initialization, in hopes of silencing some setjmp-clobber
warnings about that.  (Marking it volatile would serve too, but
its sibling variables are depending on single assignment, so let's
stick with that method.)

Discussion: https://postgr.es/m/E1l4F1z-0000cN-Lx@gemulon.postgresql.org