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.

[0] fcada52291
[1] cd51cb241a

src/fcxml.c

@@ -3689,7 +3689,10 @@ bail0:
     if (realfilename)
 	FcStrFree (realfilename);
     if (!complain)
+    {
+	FcStrBufDestroy (&reason);
 	return FcTrue;
+    }
     if (!ret && complain_again)
     {
 	if (name)