Handle zero-length Buffers in Node.js v13.2.0+

These now use nullptr internally - see
https://github.com/nodejs/node/pull/30339

docs/changelog.md

@@ -6,6 +6,8 @@ Requires libvips v8.8.1.
 
 #### v0.23.4 - TBD
 
+* Handle zero-length Buffer objects when using Node.js v13.2.0+.
+
 * Improve thread safety by using copy-on-write when updating metadata.
   [#1986](https://github.com/lovell/sharp/issues/1986)
 

src/common.cc

@@ -58,6 +58,7 @@ namespace sharp {
       v8::Local<v8::Object> buffer = AttrAs<v8::Object>(input, "buffer");
       descriptor->bufferLength = node::Buffer::Length(buffer);
       descriptor->buffer = node::Buffer::Data(buffer);
+      descriptor->isBuffer = TRUE;
       buffersToPersist.push_back(buffer);
     }
     descriptor->failOnError = AttrTo<bool>(input, "failOnError");
@@ -246,7 +247,7 @@ namespace sharp {
   std::tuple<VImage, ImageType> OpenInput(InputDescriptor *descriptor, VipsAccess accessMethod) {
     VImage image;
     ImageType imageType;
-    if (descriptor->buffer != nullptr) {
+    if (descriptor->isBuffer) {
       if (descriptor->rawChannels > 0) {
         // Raw, uncompressed pixel data
         image = VImage::new_from_memory(descriptor->buffer, descriptor->bufferLength,

src/common.h

@@ -49,6 +49,7 @@ namespace sharp {
     char *buffer;
     bool failOnError;
     size_t bufferLength;
+    bool isBuffer;
     double density;
     int rawChannels;
     int rawWidth;
@@ -64,6 +65,7 @@ namespace sharp {
       buffer(nullptr),
       failOnError(TRUE),
       bufferLength(0),
+      isBuffer(FALSE),
       density(72.0),
       rawChannels(0),
       rawWidth(0),