fix ubsan errors

Signed-off-by: Asra Ali <asraa@google.com>

lib/nghttp2_buf.c

@@ -82,8 +82,10 @@ void nghttp2_buf_reset(nghttp2_buf *buf) {
 }
 
 void nghttp2_buf_wrap_init(nghttp2_buf *buf, uint8_t *begin, size_t len) {
-  buf->begin = buf->pos = buf->last = buf->mark = begin;
-  buf->end = begin + len;
+  buf->begin = buf->pos = buf->last = buf->mark = buf->end = begin;
+  if (len) {
+    buf->end += len;
+  }
 }
 
 static int buf_chain_new(nghttp2_buf_chain **chain, size_t chunk_length,

lib/nghttp2_frame.c

@@ -818,8 +818,10 @@ int nghttp2_frame_unpack_origin_payload(nghttp2_extension *frame,
   size_t len = 0;
 
   origin = frame->payload;
-  p = payload;
-  end = p + payloadlen;
+  p = end = payload;
+  if (payloadlen) {
+    end += payloadlen;
+  }
 
   for (; p != end;) {
     if (end - p < 2) {