memset() doesn't take NULL.

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

crypto/buffer/buffer.c

@@ -128,7 +128,8 @@ size_t BUF_MEM_grow(BUF_MEM *str, size_t len)
         return (len);
     }
     if (str->max >= len) {
-        memset(&str->data[str->length], 0, len - str->length);
+        if (str->data != NULL)
+            memset(&str->data[str->length], 0, len - str->length);
         str->length = len;
         return (len);
     }
@@ -160,7 +161,8 @@ size_t BUF_MEM_grow_clean(BUF_MEM *str, size_t len)
     size_t n;
 
     if (str->length >= len) {
-        memset(&str->data[len], 0, str->length - len);
+        if (str->data != NULL)
+            memset(&str->data[len], 0, str->length - len);
         str->length = len;
         return (len);
     }