Fix a panic when failing to load some images in a diff

tools/tui/graphics/collection.go

@@ -138,7 +138,9 @@ func (self *ImageCollection) ResizeForPageSize(width, height int) {
 	ctx.Parallel(0, len(keys), func(nums <-chan int) {
 		for i := range nums {
 			img := self.images[keys[i]]
-			img.ResizeForPageSize(width, height)
+			if img.src.loaded && img.err == nil {
+				img.ResizeForPageSize(width, height)
+			}
 		}
 	})
 }
@@ -298,6 +300,7 @@ func (self *ImageCollection) LoadAll() {
 				if img.err == nil {
 					img.src.size.Width, img.src.size.Height = img.src.data.Width, img.src.data.Height
 				}
+				img.src.loaded = true
 			}
 		}
 	})