Silence undefined return value warning

clang-analyzer complains that "ret" may be returned uninitialized
which can't happen with a valid session configuration. For this to
occur, either libxml2 would have to return a bogus ChildElementCount
(return non-zero when there a actually no child nodes) _or_ the node
would have children of an unexpected type, which would be catched by
the validation performed against the XSD.

Nonetheless, the value is initialized here to silence this warning.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>