gmarkup: Add a limit on the number of attributes in an element
While the XML specification doesn’t prescribe a limit, no reasonable bit of XML is going to have more than 1000 attributes in a single XML element.
Adding a limit reduces the changes of a runaway allocation loop caused by dodgy input.
oss-fuzz#12960
Signed-off-by: Philip Withnall withnall@endlessm.com