cat-file: Fix an gcc -Wuninitialized warning

After commit cbfd5e1c ("drop some obsolete "x = x" compiler warning
hacks", 21-03-2013) removed a gcc specific hack, older versions of
gcc now issue an "'contents' might be used uninitialized" warning.
In order to suppress the warning, we simply initialize the variable
to NULL in it's declaration.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/cat-file.c

@@ -193,7 +193,7 @@ static int batch_one_object(const char *obj_name, int print_contents)
 	unsigned char sha1[20];
 	enum object_type type = 0;
 	unsigned long size;
-	void *contents;
+	void *contents = NULL;
 
 	if (!obj_name)
 	   return 1;