resources/image: Fix fill with smartcrop sometimes returning 0 bytes images

Fixes #7955

resources/images/smartcrop.go

@@ -15,6 +15,7 @@ package images
 
 import (
 	"image"
+	"math"
 
 	"github.com/disintegration/gift"
 
@@ -41,6 +42,14 @@ type imagingResizer struct {
 }
 
 func (r imagingResizer) Resize(img image.Image, width, height uint) image.Image {
+	// See https://github.com/gohugoio/hugo/issues/7955#issuecomment-861710681
+	scaleX, scaleY := calcFactorsNfnt(width, height, float64(img.Bounds().Dx()), float64(img.Bounds().Dy()))
+	if width == 0 {
+		width = uint(math.Ceil(float64(img.Bounds().Dx()) / scaleX))
+	}
+	if height == 0 {
+		height = uint(math.Ceil(float64(img.Bounds().Dy()) / scaleY))
+	}
 	result, _ := r.p.Filter(img, gift.Resize(int(width), int(height), r.filter))
 	return result
 }
@@ -71,3 +80,25 @@ func (p *ImageProcessor) smartCrop(img image.Image, width, height int, filter gi
 
 	return img.Bounds().Intersect(rect), nil
 }
+
+// Calculates scaling factors using old and new image dimensions.
+// Code borrowed from https://github.com/nfnt/resize/blob/83c6a9932646f83e3267f353373d47347b6036b2/resize.go#L593
+func calcFactorsNfnt(width, height uint, oldWidth, oldHeight float64) (scaleX, scaleY float64) {
+	if width == 0 {
+		if height == 0 {
+			scaleX = 1.0
+			scaleY = 1.0
+		} else {
+			scaleY = oldHeight / float64(height)
+			scaleX = scaleY
+		}
+	} else {
+		scaleX = oldWidth / float64(width)
+		if height == 0 {
+			scaleY = scaleX
+		} else {
+			scaleY = oldHeight / float64(height)
+		}
+	}
+	return
+}