Minor warnings fixes

Some systems insist on -Wmissing-field-initializers.  We have too many,
by design.  Fix a few easy ones.

src/hb-open-type-private.hh

@@ -289,7 +289,7 @@ template <typename Type>
 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 */

src/hb-ot-layout-private.hh

@@ -127,7 +127,7 @@ struct hb_ot_layout_lookup_accelerator_t
   }
 
   template <typename TLookup>
-  inline void fini (const TLookup &lookup)
+  inline void fini (const TLookup &lookup HB_UNUSED)
   {
   }
 

src/hb-private.hh

@@ -284,7 +284,7 @@ typedef int (*hb_compare_func_t) (const void *, const void *);
 /* arrays and maps */
 
 
-#define HB_PREALLOCED_ARRAY_INIT {0}
+#define HB_PREALLOCED_ARRAY_INIT {0, 0, NULL}
 template <typename Type, unsigned int StaticSize=16>
 struct hb_prealloced_array_t
 {