evdns: evdns_base_free(): free requests before namservers

evdns: evdns_base_free(): free requests before namservers

Otherwise we will trigger next UAF:
$ valgrind --vgdb-error=1 regress --no-fork +dns/client_fail_requests
==24733== Memcheck, a memory error detector
==24733== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==24733== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==24733== Command: regress --no-fork +dns/client_fail_requests
==24733==
==24733==
==24733== TO DEBUG THIS PROCESS USING GDB: start GDB like this
==24733==   /path/to/gdb regress
==24733== and then give GDB the following command
==24733==   target remote | /usr/lib/valgrind/../../bin/vgdb --pid=24733
==24733== --pid is optional if only one valgrind process is running
==24733==
dns/client_fail_requests: ==24733== Invalid read of size 4
==24733==    at 0x4C3352: request_finished (evdns.c:662)
==24733==    by 0x4CC8B7: evdns_base_free_and_unlock (evdns.c:4048)
==24733==    by 0x4CCAFD: evdns_base_free (evdns.c:4088)
==24733==    by 0x458E95: dns_client_fail_requests_test (regress_dns.c:2039)
==24733==    by 0x48EA5D: testcase_run_bare_ (tinytest.c:105)
==24733==    by 0x48ED3F: testcase_run_one (tinytest.c:252)
==24733==    by 0x48F67E: tinytest_main (tinytest.c:434)
==24733==    by 0x47C0DA: main (regress_main.c:461)
==24733==  Address 0x61e6f70 is 448 bytes inside a block of size 456 free'd
==24733==    at 0x4C29EAB: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==24733==    by 0x4A8F4D: event_mm_free_ (event.c:3512)
==24733==    by 0x4CC7A1: evdns_nameserver_free (evdns.c:4021)
==24733==    by 0x4CC7DC: evdns_base_free_and_unlock (evdns.c:4037)
==24733==    by 0x4CCAFD: evdns_base_free (evdns.c:4088)
==24733==    by 0x458E95: dns_client_fail_requests_test (regress_dns.c:2039)
==24733==    by 0x48EA5D: testcase_run_bare_ (tinytest.c:105)
==24733==    by 0x48ED3F: testcase_run_one (tinytest.c:252)
==24733==    by 0x48F67E: tinytest_main (tinytest.c:434)
==24733==    by 0x47C0DA: main (regress_main.c:461)
==24733==  Block was alloc'd at
==24733==    at 0x4C28C4F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==24733==    by 0x4A8D5A: event_mm_malloc_ (event.c:3437)
==24733==    by 0x4C8B96: evdns_nameserver_add_impl_ (evdns.c:2505)
==24733==    by 0x4C916D: evdns_base_nameserver_ip_add (evdns.c:2629)
==24733==    by 0x458DA3: dns_client_fail_requests_test (regress_dns.c:2031)
==24733==    by 0x48EA5D: testcase_run_bare_ (tinytest.c:105)
==24733==    by 0x48ED3F: testcase_run_one (tinytest.c:252)
==24733==    by 0x48F67E: tinytest_main (tinytest.c:434)
==24733==    by 0x47C0DA: main (regress_main.c:461)
==24733==
==24733== (action on error) vgdb me ...

Fixes: regress dns/client_fail_requests
Fixes: #269