maint: fix context_t init warning on systems without selinux

On systems without libselinux, context_t is defined as an int type,
but defined as a pointer type by SELinux.

* src/chcon.c: Init with 0 rather than NULL to avoid warning.

src/chcon.c

@@ -141,7 +141,7 @@ static int
 change_file_context (int fd, char const *file)
 {
   char *file_context = NULL;
-  context_t context IF_LINT (= NULL);
+  context_t context IF_LINT (= 0);
   char const * context_string;
   int errors = 0;