xbps-pkgdb: fix crash if db file unexistent in 'list' target.

--HG--
extra : convert_revision : a42e5a7eda63a0d121dd7f15683220c84d58b620

bin/xbps-pkgdb.c

@@ -234,6 +234,12 @@ main(int argc, char **argv)
 			usage();
 
 		dbdict = prop_dictionary_internalize_from_file(dbfile);
+		if (dbdict == NULL) {
+			printf("=> ERROR: cannot read database file (%s)\n",
+			    strerror(errno));
+			exit(EINVAL);
+		}
+
 		if (!xbps_callback_array_iter_in_dict(dbdict,
 		    "packages", xbps_list_pkgs_in_dict, NULL))
 			exit(EINVAL);