Fix for '#' in xlink:href for gradients.

This shold fix failed ref lookups in gradients.
How about a bit more checking to make sure we're parsing the right kind of hrefs?
```C
if (attr[i+1][0] == '#') {
    strncpy(grad->ref, attr[i+1] + 1, 63);  // skip leading #, keeping only the element ID
    grad->ref[63] = '\0';
}
```