BUF_strndup: tidy

Fix comment, add another overflow check, tidy style

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit de8883e11b)
(cherry picked from commit f5afe9ce3f)

crypto/buffer/buffer.h

@@ -87,8 +87,8 @@ int BUF_MEM_grow_clean(BUF_MEM *str, size_t len);
 char *BUF_strdup(const char *str);
 
 /*
- * Returns a pointer to a new string which is a duplicate of the string |str|,
- * but guarantees to never read past the first |siz| bytes of |str|.
+ * Like strndup, but in addition, explicitly guarantees to never read past the
+ * first |siz| bytes of |str|.
  */
 char *BUF_strndup(const char *str, size_t siz);