config.c: fix mmap leak when writing config

config.c: fix mmap leak when writing config

We mmap the existing config file, but fail to unmap it if we
hit an error. The function already has a shared exit path,
so we can fix this by moving the mmap pointer to the
function scope and clearing it in the shared exit.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>