Return early if RenderObject is not a meter, in  RenderThemeNix::paintMeter()

We return true if that's the case, like other ports do. This avoids a problem
with ASSERT_WITH_SECURITY_IMPLICATION FAILED: !object || object->isMeter() in
WebCore::toRenderMeter, as reported in Issue #15 on GitHub.

Reviewed by: Daker Fernandes Pinheiro