png: check magic bytes for well-formed files

Closes #$250

spec/graphics/image_magicbytes_check.rb

@@ -0,0 +1,20 @@
+require 'spec_helper'
+require 'squib/graphics/image'
+
+describe Squib do
+  context :open_png do
+
+    it 'opens a real image file' do
+      file = image_file('a.png')
+      expect(Squib.open_png(file)).to respond_to(:format) # loaded?
+    end
+
+    it 'fails on a non-PNG file' do
+      file = image_file('not_a_png.txt')
+      expect { Squib.open_png(file) }.
+        to raise_error(ArgumentError, /is not a PNG file/)
+
+    end
+
+  end
+end