export value for local -a'd temp var

[Grisha Levit]

If a temp env variable is made into a local array, the export string
for this variable gets prefixed with an extra assignment every time
the export array is rebuilt.

    $ f() {
        printenv x
        local -a x
        y=; printenv x
        y=; printenv x
        y=; printenv x
    }
    $ export y
    $ x=t f

    t
    x=t
    x=x=t
    x=x=x=t