dpkg: Use pkg_db_find_singleton() to get the dpkg package instance

src/enquiry.c

@@ -321,7 +321,10 @@ assert_version_support(const char *const *argv,
 
   modstatdb_open(msdbrw_readonly);
 
-  pkg = pkg_db_find_pkg("dpkg", NULL);
+  pkg = pkg_db_find_singleton("dpkg");
+  if (pkg == NULL)
+    internerr("more than one dpkg instance installed");
+
   switch (pkg->status) {
   case stat_installed:
   case stat_triggerspending: