Ensure image >= 3x3 before attempting trim operation

See https://github.com/libvips/libvips/issues/1392

docs/changelog.md

@@ -13,6 +13,8 @@ Requires libvips v8.8.1.
   [#1834](https://github.com/lovell/sharp/pull/1834)
   [@jaubourg](https://github.com/jaubourg)
 
+* Ensure image is at least 3x3 pixels before attempting trim operation.
+
 #### v0.23.0 - 29<sup>th</sup> July 2019
 
 * Remove `overlayWith` previously deprecated in v0.22.0.

src/operations.cc

@@ -250,6 +250,9 @@ namespace sharp {
     Trim an image
   */
   VImage Trim(VImage image, double const threshold) {
+    if (image.width() < 3 && image.height() < 3) {
+      throw VError("Image to trim must be at least 3x3 pixels");
+    }
     // Top-left pixel provides the background colour
     VImage background = image.extract_area(0, 0, 1, 1);
     if (HasAlpha(background)) {

test/unit/trim.js

@@ -58,6 +58,27 @@ describe('Trim borders', function () {
       });
   });
 
+  it('Attempt to trim 2x2 pixel image fails', function (done) {
+    sharp({
+      create: {
+        width: 2,
+        height: 2,
+        channels: 3,
+        background: 'red'
+      }
+    })
+      .trim()
+      .toBuffer()
+      .then(() => {
+        done(new Error('Expected an error'));
+      })
+      .catch(err => {
+        assert.strictEqual('Image to trim must be at least 3x3 pixels', err.message);
+        done();
+      })
+      .catch(done);
+  });
+
   describe('Invalid thresholds', function () {
     [-1, 'fail', {}].forEach(function (threshold) {
       it(JSON.stringify(threshold), function () {