Add missing checks for ipcrypt_str_to_ip16

[rgacogne]

I noticed that gcc was complaining about possibly using uninitialized memory:

In file included from /usr/include/string.h:548,
                 from ipcrypt2.c:37:
In function 'memcpy',
    inlined from 'ipcrypt_nd_encrypt_ip16' at ipcrypt2.c:1349:5,
    inlined from 'ipcrypt_nd_encrypt_ip_str' at ipcrypt2.c:1388:5:
/usr/include/bits/string_fortified.h:29:10: warning: 'ip16' may be used uninitialized [-Wmaybe-uninitialized]
   29 |   return __builtin___memcpy_chk (__dest, __src, __len,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   30 |                                  __glibc_objsize0 (__dest));
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~~~~
ipcrypt2.c: In function 'ipcrypt_nd_encrypt_ip_str':
ipcrypt2.c:1381:13: note: 'ip16' declared here
 1381 |     uint8_t ip16[16];
      |             ^~~~

which seems indeed possible if the IPv6 address could not be parsed.

[jedisct1]

Indeed, thank you!