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.

This fixes the following panic:

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x26db4]

goroutine 1 [running]:
github.com/go3d/go-collada/imp-1%2e5.xau64p(0xc20011ff80, 0x351860, 0x6, 0x0)
	/Users/Dmitri/Dmitri/^Work/^GitHub/Conception/GoLanding/src/github.com/go3d/go-collada/imp-1.5/helpers.go:349 +0xc4
github.com/go3d/go-collada/imp-1%2e5.load_SourceAccessor(0xc20011ff80, 0xc200134ac0)
	/Users/Dmitri/Dmitri/^Work/^GitHub/Conception/GoLanding/src/github.com/go3d/go-collada/imp-1.5/imploads.go:158 +0xe1
github.com/go3d/go-collada/imp-1%2e5.init_SourceAccessor(0xc20011ff80, 0xc200134ac0)
	/Users/Dmitri/Dmitri/^Work/^GitHub/Conception/GoLanding/src/github.com/go3d/go-collada/imp-1.5/impinits.go:1012 +0x59
...
```