fix creating error report when pod was ran not from project dir

I run pod search not from directory with xcode project and it there was
an error with reading podfile. Than I added check for existence of
podfile. [#177]

lib/cocoapods/command/error_report.rb

@@ -35,7 +35,7 @@ Don't forget to anonymize any private data!
 ### Podfile
 
 ```ruby
-#{Config.instance.project_podfile.read}
+#{Config.instance.project_podfile.read if Config.instance.project_podfile}
 ```