selinux: detect invalid ebitmap

When count is 0 and the highbit is not zero, the ebitmap is not
valid and the internal node is not allocated. This causes issues
when routines, like mls_context_isvalid() attempt to use the
ebitmap_for_each_bit() and ebitmap_node_get_bit() as they assume
a highbit > 0 will have a node allocated.

Signed-off-by: William Roberts <william.c.roberts@intel.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>