gvariant-parser: Add explicit unsigned-to-signed casts
Rather than prefixing unsigned numbers with unary minus operators and expecting the implicit cast to carry the correct value through, add an explicit cast to a signed type before the unary minus is applied.
In all four cases, an overflow check has already been done.
Signed-off-by: Philip Withnall firstname.lastname@example.org
Closes #1655 (closed)