lxc/utils: bugfix freed pointer return value

We allocate a pointer and save this address in a static variable. After
this we freed this pointer and return.

Here a cuttout of a valgrind report:

	[...]
	==11568== Invalid read of size 1
	==11568==    at 0x4C2D524: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
	==11568==    by 0x5961C9B: puts (in /usr/lib/libc-2.20.so)
	==11568==    by 0x400890: main (lxc_config.c:73)
	==11568==  Address 0x6933e21 is 1 bytes inside a block of size 32 free'd
	==11568==    at 0x4C2B200: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
	==11568==    by 0x4E654F2: lxc_global_config_value (utils.c:415)
	==11568==    by 0x4E92177: lxc_get_global_config_item (lxccontainer.c:2287)
	==11568==    by 0x400883: main (lxc_config.c:71)
	[...]

Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>