[IOS] Fix popen crash during xbmc starting up, it may not work on ios

I found it always crash my iphone4 ios 5.1.1 without debugger attached.

the crash log: http://xbmclogs.com/show.php?id=4946

anyway, even with debugger, the popen(uname...) just fails, but the log not produced since where the code called is earlier than the log file be opened.

so here's the fix, with this, I feel the startup of xbmc is much faster than ever.