Add SDL 2.0 compatibility.

Use 0 instead of NULL for pointers, the type is a int in SDL 2.0.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56429 75b3775d-3801-0410-9e12-e2e43121521d
Remove version_info::operator=().

It makes cppcheck happy and this one does the same job as one generated
by the compiler.

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56430 75b3775d-3801-0410-9e12-e2e43121521d