Handle template error gracefully when line number cannot be derived. Closes #11327 [Nex3]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@9027 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/lib/action_view/template_error.rb

@@ -41,8 +41,9 @@ module ActionView
 
       indent = ' ' * indentation
       line_counter = start_on_line
-
-      source_code[start_on_line..end_on_line].sum do |line|
+      return unless source_code = source_code[start_on_line..end_on_line] 
+      
+      source_code.sum do |line|
         line_counter += 1
         "#{indent}#{line_counter}: #{line}"
       end