slirp: avoid IN6_IS_ADDR_UNSPECIFIED(), rather use in6_zero()

slirp: avoid IN6_IS_ADDR_UNSPECIFIED(), rather use in6_zero()

Host: Mac OS 10.12.5
Compiler: Apple LLVM version 8.1.0 (clang-802.0.42)

  slirp/ip6_icmp.c:80:38: warning: taking address of packed member 'ip_src' of class or
        structure 'ip6' may result in an unaligned pointer value
        [-Waddress-of-packed-member]
              IN6_IS_ADDR_UNSPECIFIED(&ip->ip_src)) {
                                       ^~~~~~~~~~
  /usr/include/netinet6/in6.h:238:42: note: expanded from macro 'IN6_IS_ADDR_UNSPECIFIED'
          ((*(const __uint32_t *)(const void *)(&(a)->s6_addr[0]) == 0) && \
                                                  ^

Reported-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>