Dubious coding in nbtinsert.c

Buildfarm member curculio, which doesn't usually produce
uninitialized-variable warnings, is showing one here:

nbtinsert.c: In function '_bt_doinsert':
nbtinsert.c:411: warning: 'curitemid' may be used uninitialized in this function
nbtinsert.c:411: note: 'curitemid' was declared here

I can see its point: curitemid is set only if !inposting.
While the first two uses of the value are clearly reached
only if !inposting, it's FAR from clear that it's impossible
to reach "ItemIdMarkDead(curitemid);" without a valid value.
Could you clean that up?

			regards, tom lane