gvariant: Fix error handling for parsing Unicode escapes
When parsing an escaped Unicode character in a text format GVariant string, such as '\U0001F415', the code uses g_ascii_strtoull(). This, unexpectedly, accepts minus signs, which can cause an assertion failure when input like '\u-FF4' is presented for parsing.
Validate that there are no leading sign characters when parsing.
This shouldn’t be considered a security bug, because the GVariant text format parser should not be used on untrusted input.
Signed-off-by: Philip Withnall email@example.com