Don't allocate more than is needed in BUF_strndup()

(Imported from upstream's 4ceb430a468e8226175aa3f169c0e746877c17e1,
4f7236edc7d5c384bdb148faf7b23f887cf18f69 and
ed693e43329383c0d68455d83778cdc9748a074d)

crypto/buf/buf.h

diff --git a/crypto/buf/buf.h b/crypto/buf/buf.h
index 4cfeee4..d1e63f2 100644
--- a/crypto/buf/buf.h
+++ b/crypto/buf/buf.h
@@ -89,6 +89,11 @@
 /* BUF_strdup returns an allocated, duplicate of |str|. */
 char *BUF_strdup(const char *str);
 
+/* BUF_strnlen returns the number of characters in |str|, excluding the NUL
+ * byte, but at most |max_len|. This function never reads more than |max_len|
+ * bytes from |str|. */
+size_t BUF_strnlen(const char *str, size_t max_len);
+
 /* BUF_strndup returns an allocated, duplicate of |str|, which is, at most,
  * |size| bytes. The result is always NUL terminated. */
 char *BUF_strndup(const char *str, size_t size);