handle_control_postdescriptor() modifies a const char

handle_control_postdescriptor() in src/or/control.c modifies the const char body. This is a pretty bad practice and might causes errors with non writable memory segments.

static int handle_control_postdescriptor(control_connection_t *conn, 
                                         uint32_t len, const char *body) {
    char *cp = memchr(body, '\n', len);
    smartlist_t *args = smartlist_new();
    tor_assert(cp);*cp++ = '\0';