BUG/MINOR: haproxy: fix out-of-scope tmpdir usage in do_chroot()

BUG/MINOR: haproxy: fix out-of-scope tmpdir usage in do_chroot()

Coverity (CID 1660133, GH #3470) reports that dir may point to the
block-local tmpdir[] array after it has gone out of scope, since dir
is read again in the "Cannot chroot()" alert past the end of the
"chroot auto" if-block. Move tmpdir's declaration to function scope
so its lifetime covers every use of dir.

Must be backported to 3.4.