cgxget: Fix clang warning

cgxget: Fix clang warning

Fix the following clang warning by explicitily initializing disk_cg to
NULL.

	tools/cgxget.c:903:6: warning: variable 'disk_cg' is used uninitialized
	whenever 'if' condition is true [-Wsometimes-uninitialized]
	  903 |         if (!cg || !(*cg)) {
	      |             ^~~~~~~~~~~~~
	tools/cgxget.c:940:6: note: uninitialized use occurs here
	  940 |         if (disk_cg)
	      |             ^~~~~~~
	tools/cgxget.c:903:2: note: remove the 'if' if its condition is always
	false
	  903 |         if (!cg || !(*cg)) {
	      |         ^~~~~~~~~~~~~~~~~~~~
	  904 |                 ret = ECGINVAL;
	      |                 ~~~~~~~~~~~~~~~
	  905 |                 goto out;
	      |                 ~~~~~~~~~
	  906 |         }
	      |         ~

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>