Picture Ingredient: Fix NaN error with free height

When indicating that a picture can be cropped, but needs to have a
certain height or width but leaves the other dimension free, current
Alchemy throws a `NaN` error because of float division by Zero.

This commit handles this case by returning nil if the calculated mask is
Not a Number.

(cherry picked from commit 1fc477a)

app/models/concerns/alchemy/picture_thumbnails.rb

@@ -115,6 +115,8 @@ def default_crop_size
       return nil unless settings[:crop] && settings[:size]
 
       mask = inferred_dimensions_from_string(settings[:size])
+      return if mask.nil?
+
       zoom = thumbnail_zoom_factor(mask)
       return nil if zoom.zero?
 
@@ -150,6 +152,8 @@ def inferred_dimensions_from_string(string)
       width, height = dimensions_from_string(string)
       ratio = image_file_width.to_f / image_file_height.to_i
 
+      return if ratio.nan?
+
       if width.zero? && ratio.is_a?(Float)
         width = height * ratio
       end

lib/alchemy/test_support/having_picture_thumbnails_examples.rb

@@ -385,6 +385,26 @@
           size: "160x120"
         )
       end
+
+      context "with settings indicating free height" do
+        let(:settings) do
+          {
+            crop: true,
+            size: "800x"
+          }
+        end
+
+        it "returns default thumbnail options" do
+          is_expected.to eq(
+            crop: true,
+            crop_from: nil,
+            crop_size: nil,
+            flatten: true,
+            format: "jpg",
+            size: "160x120"
+          )
+        end
+      end
     end
   end