memset() doesn't take NULL.

Reviewed-by: NEmilia Käsper <emilia@openssl.org>

crypto/buffer/buffer.c

@@ -128,6 +128,7 @@ size_t BUF_MEM_grow(BUF_MEM *str, size_t len)
         return (len);
     }
     if (str->max >= len) {
+        if (str->data != NULL)
             memset(&str->data[str->length], 0, len - str->length);
         str->length = len;
         return (len);
@@ -160,6 +161,7 @@ size_t BUF_MEM_grow_clean(BUF_MEM *str, size_t len)
     size_t n;
 
     if (str->length >= len) {
+        if (str->data != NULL)
             memset(&str->data[len], 0, str->length - len);
         str->length = len;
         return (len);