tools: initialize a variable

tools: initialize a variable

Squashing
warning: ‘sym’ may be used uninitialized in this function

Here was no real usage of uninitialized variable, but
compiler still complains.

Pointer sym is allocated when flag with_val is set:

    if (with_val) {
        sym = agattr(g, AGEDGE, "len", "1");
    }

So just squash the warning by setting the
pointer to NULL and test it in guard condition
instead of testing the flag:

            if (sym && val) {
                sprintf(buf, "%f", val[j]);
                agxset(e, sym, buf);
            }