Fixed accidental record crypto computations overwrite on double prepare

Author: ic0ns

TLS-Core/src/main/java/de/rub/nds/tlsattacker/core/record/Record.java

@@ -134,6 +134,8 @@ public void setComputations(RecordCryptoComputations computations) {
 
     @Override
     public void prepareComputations() {
-        this.computations = new RecordCryptoComputations();
+        if (computations == null) {
+            this.computations = new RecordCryptoComputations();
+        }
     }
 }