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

@@ -6,22 +6,31 @@ describe Squib::Graphics::CairoContextWrapper do
   let(:cairo) { double(Cairo::Context) }
   subject     { Squib::Graphics::CairoContextWrapper.new(cairo) }
 
-  it 'passes on colors as normal' do
-    expect(cairo).to receive(:set_source_color).with('blue')
-    subject.set_source_squibcolor('blue')
-  end
+  context '#set_source_squibcolor' do
 
-  it 'passes on color symbols as normal' do
-    expect(cairo).to receive(:set_source_color).with(:blue)
-    subject.set_source_squibcolor(:blue)
-  end
+    it 'passes on colors as normal' do
+      expect(cairo).to receive(:set_source_color).with('blue')
+      subject.set_source_squibcolor('blue')
+    end
+
+    it 'passes on color symbols as normal' do
+      expect(cairo).to receive(:set_source_color).with(:blue)
+      subject.set_source_squibcolor(:blue)
+    end
+
+    it 'passes on color hashes' do
+      expect(cairo).to receive(:set_source_color)
+        .with('#aabbccdd')
+      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
 
-  it 'passes on color hashes' do
-    expect(cairo).to receive(:set_source_color)
-      .with('#aabbccdd')
-    subject.set_source_squibcolor('#aabbccdd')
   end
 
+
   context 'regex variations for linear gradients' do
     before(:each) do
       dbl = double(Cairo::LinearPattern)