Add nil check for colors, just in case

Probably not possible, but provides a more descriptive error than before anyway.

Fixes #84.

lib/squib/graphics/cairo_context_wrapper.rb

@@ -30,6 +30,7 @@ module Squib
       # :nodoc:
       # @api private
       def set_source_squibcolor(arg)
+        raise 'nil is not a valid color' if arg.nil?
         if match = arg.match(LINEAR_GRADIENT)
           x1, y1, x2, y2 = match.captures
           linear = Cairo::LinearPattern.new(x1.to_f, y1.to_f, x2.to_f, y2.to_f)

spec/graphics/cairo_context_wrapper_spec.rb

@@ -6,6 +6,8 @@ describe Squib::Graphics::CairoContextWrapper do
   let(:cairo) { double(Cairo::Context) }
   subject     { Squib::Graphics::CairoContextWrapper.new(cairo) }
 
+  context '#set_source_squibcolor' do
+
     it 'passes on colors as normal' do
       expect(cairo).to receive(:set_source_color).with('blue')
       subject.set_source_squibcolor('blue')
@@ -22,6 +24,13 @@ describe Squib::Graphics::CairoContextWrapper do
       subject.set_source_squibcolor('#aabbccdd')
     end
 
+    it 'raises on nil' do
+      expect { subject.set_source_squibcolor(nil) }.to raise_error('nil is not a valid color')
+    end
+
+  end
+
+
   context 'regex variations for linear gradients' do
     before(:each) do
       dbl = double(Cairo::LinearPattern)