prevent fclose(NULL) on dbminit() failure

When fed a bad database, dbminit() could call fclose() on a NULL pointer.
Bug reported by the Mayhem team at CMU.

--- a/dbz/dbz.c
+++ b/dbz/dbz.c
@@ -873,7 +873,8 @@ char *name;
 	/* pick up configuration */
 	if (getconf(dirf, pagf, &conf) < 0) {
 		DEBUG(("dbminit: getconf failure\n"));
-		(void) fclose(basef);
+		if (basef != NULL)
+			(void) fclose(basef);
 		(void) fclose(pagf);
 		(void) fclose(dirf);
 		free((POINTER)pagfname);
