Fix assertion in FcFini()

Due to the unproper initialization of `latest_mtime', the duplicate caches
was still in fcCacheChains with no references. which means no one frees
them. thus, the memory leak was happened.

Fixes fontconfig/fontconfig#227
3 jobs for issue/227 in 5 minutes and 41 seconds
Status Job ID Name Coverage
  Build
passed #1736066
mingw-build

00:03:19

passed #1736064
shared-build

00:05:41

passed #1736065
static-build

00:05:39