Fix leak of dpkg cache when dpkginfo_init is called multiple times

In the dpkginfo probe, the dpkg cache was allocated in dpkginfo_init.
However, dpkginfo_init can be called multiple times, leading the
cgCache pointer to be overridden and leaking the dpkg cache.

src/OVAL/probes/unix/linux/dpkginfo-helper.cxx

@@ -127,6 +127,10 @@ void dpkginfo_free_reply(struct dpkginfo_reply_t *reply)
 
 int dpkginfo_init()
 {
+ if (cgCache != NULL) {
+ return 0;
+ }
+
  cgCache = new pkgCacheFile;
  if (_init_done == 0)
  if (opencache() != 1) {