sd-resolve: workaround for structured initialization to nested structs

sd-resolve: workaround for structured initialization to nested structs

When a nested struct is initialized by structured initializer, then
padding space is not cleared by zero. So, before setting values,
this makes explicitly set zero including padding.

This fixes the following false positive warning by valgrind:
```
==492== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==492==    at 0x56D0CF7: sendmsg (in /usr/lib64/libpthread-2.27.so)
==492==    by 0x4FDD3C5: sd_resolve_getaddrinfo (sd-resolve.c:975)
==492==    by 0x110B9E: manager_connect (timesyncd-manager.c:879)
==492==    by 0x10B729: main (timesyncd.c:165)
==492==  Address 0x1fff0008f1 is on thread 1's stack
==492==  in frame #1, created by sd_resolve_getaddrinfo (sd-resolve.c:928)
==492==
```