Fix nil pointer dereference in xau64p().

Note that there are several other funcs here that try to assign to nil pointer (e.g., xbp(), xf64p(), xi64p(), xabp(), xu64p()). Unless I'm off base here, they should be fixed too.
Merge pull request #2 from shurcooL/patch-1

Fix nil pointer dereference in xau64p().