Fix Build#append_trace method usage when trace file doesn't exists yet

app/models/ci/build.rb

@@ -253,7 +253,7 @@ module Ci
     def append_trace(trace_part, offset)
       recreate_trace_dir
 
-      File.truncate(path_to_trace, offset)
+      File.truncate(path_to_trace, offset) if File.exist?(path_to_trace)
       File.open(path_to_trace, 'a') do |f|
         f.write(trace_part)
       end

lib/ci/api/builds.rb

@@ -71,8 +71,9 @@ module Ci
           content_range = request.headers['Content-Range']
           content_range = content_range.split('-')
 
-          unless build.trace_length == content_range[0].to_i
-            return error!('416 Range Not Satisfiable', 416, { 'Range' => "0-#{build.trace_length}" })
+          current_length = build.trace_length
+          unless current_length == content_range[0].to_i
+            return error!('416 Range Not Satisfiable', 416, { 'Range' => "0-#{current_length}" })
           end
 
           build.append_trace(request.body.read, content_range[0].to_i)