MDEV-19882 pam v2: auth_pam_tool truncates passwords that are not null-terminated

Don't assume that passwords (and other conv replies) are zero-terminated.
If they are, though, strndup() down below will take care of that.

plugin/auth_pam/auth_pam_tool.c

@@ -33,7 +33,7 @@ static int roundtrip(struct param *param, const unsigned char *buf,
   if (write(1, &b, 1) < 1 || write_string(1, buf, buf_len))
     return -1;
   *pkt= (unsigned char *) param->buf;
-  return read_string(0, (char *) param->buf, (int) sizeof(param->buf)) - 1;
+  return read_string(0, (char *) param->buf, (int) sizeof(param->buf));
 }
 
 typedef struct st_mysql_server_auth_info