Protect against nil return codes in rakefiles

examples/example_3/rakefile_helper.rb

@@ -141,7 +141,7 @@ module RakefileHelpers
     report command_string
     output = `#{command_string}`.chomp
     report(output) if verbose && !output.nil? && !output.empty?
-    if !$?.exitstatus.zero? && raise_on_fail
+    if !$?.nil? && !$?.exitstatus.zero? && raise_on_fail
       raise "Command failed. (Returned #{$?.exitstatus})"
     end
     output

test/rakefile_helper.rb

@@ -173,7 +173,7 @@ module RakefileHelpers
     report command_string if $verbose
     output = `#{command_string}`.chomp
     report(output) if $verbose && !output.nil? && !output.empty?
-    raise "Command failed. (Returned #{$?.exitstatus})" if !$?.exitstatus.zero? && !ok_to_fail
+    raise "Command failed. (Returned #{$?.exitstatus})" if !$?.nil? && !$?.exitstatus.zero? && !ok_to_fail
     output
   end