Fixes for several memory leaks

[Lukas Fleischer]

On Sat, Apr 02, 2011 at 10:18:44PM +0200, Lukas Fleischer wrote:
> Hi,
> 
> I created patches for a whole bunch of memory leaks (probably around 100
> leaks altogether) which mostly came up due to assigning newly allocated
> strings (using xstrdupn() in most cases) to configuration strings
> without free()'ing the previous pointer at all. Note that this doesn't
> cover everything by far. There's a lot of more leaks (mainly with repo,
> ref and commit handling) which I didn't have time to fix yet tho.
> 
> Please review and test in detail before pushing. I tried to cover
> everything but I might have missed some tricky parts. I also only did
> superficial tests...
> 
> Clone URL of my cgit working tree is [1], web interface of the "wip"
> branch can be found on [2].
> 
> [1] git://git.cryptocrack.de/cgit.git
> [2] http://git.cryptocrack.de/cgit.git/?h=wip

Added some null pointer dereference fixes.