Improve parsing of "r"

Albert Astals Cidrequested to merge
aacid/qtsvg:improve_r_parsing into kde/5.15

Negative r values are illegal, and zero means empty for circles.

Pick-to: 6.1

Change-Id: Icb1d932f35909f71dafe1ee69eb2250eeb1bb2ad

Reviewed-by: default avatarMårten Nordheim marten.nordheim@qt.io

(cherry picked from commit 4a88e194)

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