don't overwrite the dns_master_loadfile() result before calling zone_postload()

don't overwrite the dns_master_loadfile() result before calling zone_postload()

if "rndc reload" fails, the result code is supposed to be passed to
zone_postload, but for inline-signing zones, the result can be
overwritten first by a call to the ZONE_TRYLOCK macro. this can lead
to the partially-loaded unsigned zone being synced over to the signed
zone instead of being rejected.

(cherry picked from commit 0b792bd37b9b6a1cc5914ab95987359fda972ff2)