LinkKey hash function always returns 0
#### Description
The `hashCode()` method in [LinkKey](https://github.com/naver/pinpoint/blob/master/web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java#L106) class always returns 0. resulting in N * N equals comparisons for caller/callee pairs when querying server map.
web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java
web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java
diff --git a/web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java b/web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java
index 58bffa661265..eb92600afcfe 100644
--- a/web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java
+++ b/web/src/main/java/com/navercorp/pinpoint/web/vo/LinkKey.java
@@ -103,7 +103,7 @@ public boolean equals(Object o) {
     @Override
     public int hashCode() {
         final int hash = this.hash;
-        if (hash == 0) {
+        if (hash != 0) {
             return hash;
         }
         int result = fromApplication.hashCode();
