Add specs for #read_asciiz and fix bugs there

lib/rex/image_source/image_source.rb

@@ -29,8 +29,12 @@ class ImageSource
     # FIXME, make me better
     string = ''
     loop do
+      begin
         char = read(offset, 1)
-      break if char == "\x00"
+      rescue RangeError
+        break
+      end
+      break if char.nil? || char == "\x00"
       offset += 1
       string << char
     end

spec/lib/rex/image_source/disk_spec.rb

@@ -17,6 +17,8 @@ describe Rex::ImageSource::Disk do
     described_class.new(file)
   end
 
+  it_should_behave_like 'Rex::ImageSource::ImageSource'
+
   describe "#initialize" do
     subject(:disk_class) do
       described_class.allocate

spec/lib/rex/image_source/memory_spec.rb

@@ -11,6 +11,8 @@ describe Rex::ImageSource::Memory do
     described_class.new(raw_data)
   end
 
+  it_should_behave_like 'Rex::ImageSource::ImageSource'
+
   describe "#initialize" do
     subject(:memory_class) do
       described_class.allocate