Fix gcc 8.3 warning (stringop-truncation)

[karel-m]

Warning related to ssh_encode_sequence_multi.c

In file included from src/headers/tomcrypt.h:22,
                 from src/headers/tomcrypt_private.h:10,
                 from src/misc/ssh/ssh_encode_sequence_multi.c:9:
src/misc/ssh/ssh_encode_sequence_multi.c: In function 'ssh_encode_sequence_multi':
src/headers/tomcrypt_custom.h:49:18: error: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
 #define XSTRNCPY strncpy
src/misc/ssh/ssh_encode_sequence_multi.c:124:13: note: in expansion of macro 'XSTRNCPY'
             XSTRNCPY((char *)out, sdata, size);
             ^~~~~~~~
src/misc/ssh/ssh_encode_sequence_multi.c:121:20: note: length computed here
             size = strlen(sdata);
                    ^~~~~~~~~~~~~