gvariant-parser: Fix parsing of G_MININT* values in GVariant text format
And add tests.
There wasn’t actually a bug on x86_64 before, but it was making use of undefined behaviour, and hence triggering ubsan warnings. Make the code more explicit, and avoid undefined behaviour.
oss-fuzz#12686
Signed-off-by: Philip Withnall withnall@endlessm.com