Don't pass NULL to memcpy

lib/nghttp2_session.c

@@ -5640,10 +5640,12 @@ ssize_t nghttp2_session_mem_recv(nghttp2_session *session, const uint8_t *in,
 
       readlen = inbound_frame_payload_readlen(iframe, in, last);
 
-      iframe->lbuf.last = nghttp2_cpymem(iframe->lbuf.last, in, readlen);
+      if (readlen > 0) {
+        iframe->lbuf.last = nghttp2_cpymem(iframe->lbuf.last, in, readlen);
 
-      iframe->payloadleft -= readlen;
-      in += readlen;
+        iframe->payloadleft -= readlen;
+        in += readlen;
+      }
 
       DEBUGF(fprintf(stderr, "recv: readlen=%zu, payloadleft=%zu\n", readlen,
                      iframe->payloadleft));