[Backport] CVE-2021-3541 libxml2: Exponential entity expansion attack bypasses...

[Backport] CVE-2021-3541 libxml2: Exponential entity expansion attack bypasses all existing protection mechanisms

Manual backport of patch originally committed at
https://gitlab.gnome.org/GNOME/libxml2/-/commit/8598060bacada41a0eb09d95c97744ff4e428f8e

:
Patch for security issue CVE-2021-3541

This is relapted to parameter entities expansion and following
the line of the billion laugh attack. Somehow in that path the
counting of parameters was missed and the normal algorithm based
on entities "density" was useless.

Change-Id: I81d1ab274ae80a9e0e0890dada92d3f09584e4e7
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>