BUILD/MINOR: 51d: fix warning when building with 51Degrees release version 3.2.12.12

The warning appears when building with 51Degrees release that uses a new
Hash Trie algorithm (release version 3.2.12.12):

src/51d.c: In function init_51degrees:
src/51d.c:566:2: warning: enumeration value DATA_SET_INIT_STATUS_TOO_MANY_OPEN_FILES not handled in switch [-Wswitch]
  switch (_51d_dataset_status) {
  ^

This patch can be backported in 1.7.
diff --git a/src/51d.c b/src/51d.c
index 421c0de..1f1a5f9 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -605,6 +605,9 @@
 		case DATA_SET_INIT_STATUS_NOT_SET:
 			chunk_printf(temp, "Data set not initialised.");
 			break;
+		default:
+			chunk_printf(temp, "Other error.");
+			break;
 	}
 	if (_51d_dataset_status != DATA_SET_INIT_STATUS_SUCCESS) {
 		if (temp->len)