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.

oss-fuzz#11576

Signed-off-by: Philip Withnall <withnall@endlessm.com>