mod_dav: Fix security issue in unreleased MS-WDV support

[notroj]

mod_dav: Fix security issue in unreleased MS-WDV support:

* modules/dav/main/ms_wdv.c (mswdv_combined_proppatch):
  The MS-WDV combined PROPPATCH handler reads a 16-byte hex length
  prefix from the request body and uses it directly for memory
  allocation without bounds checking. An attacker can specify an
  extremely large value to trigger OOM and crash the worker process.

  This patch validates the parsed length against LimitXMLRequestBody
  and APR_SIZE_MAX before allocation.

Reported by: Pavel Kohout, Aisle Research, www.aisle.com
Submitted by: Pavel Kohout, jorton