... when using relative units (em, ex)

This is because for nodes of type textnode, getFontSizeNumber
is called from SvgCharacterNode::createSimpleTextPrimitive
and from SvgNode::getCurrentFontSize(), so avoid the second call.

Change-Id: Ibd418708a572e3c1643164a900fac7e7481afe81
Reviewed-on: https://gerrit.libreoffice.org/21801Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>