Resolver: fixed possible use-after-free while resolving SRV. Resolving an SRV record includes resolving its host names in subrequests. Previously, if memory allocation failed while reporting a subrequest result after receiving a response from a DNS server, the SRV resolve handler was called immediately with the NGX_ERROR state. However, if the SRV record included another copy of the resolved name, it was reported once again. This could trigger the use-after-free memory access after SRV resolve handler freed the resolve context by calling ngx_resolve_name_done(). Now the SRV resolve handler is called only when all its subrequests are completed.