Crash in there while accessing an evidently deleted DAVResourceAccess
instance with about 4 other threads also in various webdav_ucp::Content
methods.

The problem is apparently that the "outer" getResourceType() passes the
m_xResAccess to the "inner" getResourceType(), which accesses it
without locking the mutex, while another thread resets m_xResAccess
and thereby deletes the current instance.

(regression from 0c350011)

Change-Id: I1c67021c536e303d766c7ff93fb71e40f991f3af