speedo: Fix a minor memleak in the installer

* build-aux/speedo/w32/g4wihelp.c (path_remove): Free path_new on
early return.

It's a weird condition in a once run function in a throwaway
process but -- yeah. It's a memleak and static analysis can
see it.

• GnuPG-Bug-Id: T3197
• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>