Add hb_memcpy() that does len=0 check

src/hb-algs.hh

@@ -760,6 +760,14 @@ static inline unsigned int ARRAY_LENGTH (const Type (&)[n]) { return n; }
 #define ARRAY_LENGTH_CONST(__array) ((signed int) (sizeof (__array) / sizeof (__array[0])))
 
 
+static inline void *
+hb_memcpy (void *__restrict dst, const void *__restrict src, size_t len)
+{
+  /* It's illegal to pass 0 as size to memcpy. */
+  if (unlikely (!len)) return dst;
+  return memcpy (dst, src, len);
+}
+
 static inline int
 hb_memcmp (const void *a, const void *b, unsigned int len)
 {

src/hb-set.hh

@@ -489,12 +489,8 @@ struct hb_set_t
       return;
     population = other.population;
 
-    if (!count)
-      // memcpy is not necessary if the vectors are zero length. This avoids possibly
-      // passing nullptr to memcpy.
-      return;
-    memcpy ((void *) pages, (const void *) other.pages, count * pages.item_size);
-    memcpy ((void *) page_map, (const void *) other.page_map, count * page_map.item_size);
+    hb_memcpy ((void *) pages, (const void *) other.pages, count * pages.item_size);
+    hb_memcpy ((void *) page_map, (const void *) other.page_map, count * page_map.item_size);
   }
 
   bool is_equal (const hb_set_t &other) const