NULL terminate server_sign_algorithms string (#669)

files: packet.c, libssh2_priv.h

notes:
* Fix heap buffer overflow in _libssh2_key_sign_algorithm

When allocating `session->server_sign_algorithms` which is a `char*` is is important to also allocate space for the string-terminating null byte at the end and make sure the string is actually null terminated.

Without this fix, the `strchr()` call inside the `_libssh2_key_sign_algorithm` (line 1219) function will try to parse the string and go out of buffer on the last invocation.

Credit: tihmstar
Co-authored-by: Will Cosgrove <will@panic.com>

src/libssh2_priv.h

@@ -642,7 +642,6 @@ struct _LIBSSH2_SESSION
 
     /* public key algorithms accepted as comma separated list */
     char *server_sign_algorithms;
-    size_t server_sign_algorithms_len;
 
     /* key signing algorithm preferences -- NULL yields server order */
     char *sign_algo_prefs;