fix undefined variable error in uncompress_ast

[vchuravy]

Introduced by #31191

I managed to trigger this by executing:

Base.Threads.@threads for i in 1:Base.Threads.nthreads()
           if Base.Threads.threadid() == 1
             tasks = Task[]
             for i in 1:10
               t = Task(()->println("tid: ", Base.Threads.threadid()))
               t.sticky = false
               push!(tasks, t)
               schedule(t)
             end
             Base.sync_end(tasks)
           end
        end

Or by compiling the sysimage with JULIA_NUM_THREADS=4 set.

@vtjnash what would be the expected codepath to trigger this method?