fuzz_ip6: dynamically grow the fuzzbuf
fuzz_ip6: dynamically grow the fuzzbuf

Otherwise, some option combinations can strcat beyond the fixed-size
fuzzbuf, such as this invocation of fuzzbuf:

sudo ./fuzz_ip6 -4  -I -S -D -R -J -F ens5 2011:1::2

vanhauser-thc/thc-ipv6,vanhauser-thc/thc-ipv6,vanhauser-thc/thc-ipv6