Minor warnings fixes

Some systems insist on -Wmissing-field-initializers.  We have too many,
by design.  Fix a few easy ones.
diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 046df97..475187b 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -289,7 +289,7 @@
 struct Sanitizer
 {
   static hb_blob_t *sanitize (hb_blob_t *blob) {
-    hb_sanitize_context_t c[1] = {{0}};
+    hb_sanitize_context_t c[1] = {{0, NULL, NULL, false, 0, NULL}};
     bool sane;
 
     /* TODO is_sane() stuff */