Fix leak of `reason` in _FcConfigParse when not complaining
In "Do not return FcFalse from FcConfigParseAndLoad*() if complain is
set to false" [0], _FcConfigParse was updated to return FcTrue early if
it was instructed not to complain on errors. In "Take effect sysroot
functionality to the default config file" [1] the error messages were
factored into a reason which was allocated. Be sure to free this
reason in the case of returning FcTrue early.
Discovered with a leak checker while attempting to configure in interesting ways.