Fix Clang 3.7 compatibility
Fix Clang 3.7 compatibility

Apparently Clang 3.7 implements C++ DR 1748 that makes placement new with null
pointer undefined behavior. Which renders all C++ programs that rely on this
invalid. Which includes pugixml.

This is not very likely to happen in the wild because the allocations that are
subject to this in pugixml are relatively small, but tests break because of
this.

Fix the issue by adding null pointer checks (that are completely redundant in
all current compilers except Clang 3.7 but it's not like there is another
option).

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml