svg: fix invalid pointer comparisons in style parsing

A style declaration with a missing colon led to relational comparisons
against nullptr in xmlParseW3CAttribute() and svgUtilUnskipWhiteSpace(),
which is undefined behavior and aborts under the GCC/Clang
pointer-compare sanitizer.

Replace missing separators with the end pointer so every comparison
stays within the buffer, and skip the backward whitespace scan for an
empty range.

issue: https://github.com/thorvg/thorvg/issues/4641