ebitmap: detect invalid bitmap

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>

libsepol/src/ebitmap.c

@@ -394,6 +394,10 @@ int ebitmap_read(ebitmap_t * e, void *fp)
 		     e->highbit, MAPSIZE);
 		goto bad;
 	}
+
+	if (e->highbit && !count)
+		goto bad;
+
 	l = NULL;
 	for (i = 0; i < count; i++) {
 		rc = next_entry(buf, fp, sizeof(uint32_t));