Remove alloca() call in get_dns_addr_resolv_conf()

The ALLOCA(3) man-page mentions its "use is discouraged".

For now get_dns_addr_resolv_conf() is called with pointer to a in_addr/in6_addr structure, and its size. Declare a union of these structures on the stack, able to hold both of them. This allows us to remove the alloca() call, keeping the buffer on the stack.

Add an assertion in the unlikely case another inet address is handled by this function.

Signed-off-by: Philippe Mathieu-Daudé philmd@redhat.com Message-Id: 20210507133212.1952121-1-philmd@redhat.com