Check return code of xQueryTree and throw on error.

[ghost]

Fixes issue #56, where uninitialized memory would be accessed
after xQueryTree failure.

[ghost]

Any chance to get this merged, or reason not to?

Currently when XQueryTree fails, the *_return values are uninitialized, and
peekArray will read some arbitrary number of bytes from arbitrary location.
The uninitialized value of *children_return will be used with xFree as well.

In case you are really lucky and everything happens to be zeroed, you might get
an empty list instead, otherwise this is constant source of segmentation faults.