Fix signed/unsigned comparison warning

Author: zbjornson

CHANGELOG.md

@@ -15,6 +15,7 @@ project adheres to [Semantic Versioning](http://semver.org/).
 * Fix BMP issues. (#1497)
 * Update typings to support jpg and addPage on NodeCanvasRenderingContext2D (#1509)
 * Fix assertion failure when using Visual Studio Code debugger to inspect Image prototype (#1534)
+* Fix signed/unsigned comparison warning introduced in 2.6.0, and function cast warnings with GCC8+
 
 2.6.1
 ==================

src/Canvas.cc

@@ -412,7 +412,7 @@ NAN_METHOD(Canvas::ToBuffer) {
   if (info[0]->StrictEquals(Nan::New<String>("raw").ToLocalChecked())) {
     cairo_surface_t *surface = canvas->surface();
     cairo_surface_flush(surface);
-    if (canvas->nBytes() > node::Buffer::kMaxLength) {
+    if (static_cast<uint32_t>(canvas->nBytes()) > node::Buffer::kMaxLength) {
       Nan::ThrowError("Data exceeds maximum buffer length.");
       return;
     }