NumericSupport: fix serialization of BigInteger

Author: cosminbasca

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/util/NumericSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2021, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * The Universal Permissive License (UPL), Version 1.0
@@ -315,7 +315,7 @@ public void putBigInteger(byte[] buffer, int index, BigInteger value, int numByt
         final byte[] src = value.toByteArray();
         int srcIndex = 0;
         int srcBytes = src.length;
-        if (src.length > numBytes) {
+        if (srcBytes > numBytes) {
             for (int i = 0; i < src.length; i++) {
                 srcIndex = i;
                 if (src[i] != 0) {
@@ -326,6 +326,13 @@ public void putBigInteger(byte[] buffer, int index, BigInteger value, int numByt
             if (srcBytes > numBytes) {
                 throw OverflowException.INSTANCE;
             }
+        } else if (srcBytes < numBytes) {
+            // perform sign extension
+            if (value.signum() < 0) {
+                for (int i = 0; i < numBytes - srcBytes; i++) {
+                    buffer[i] = -1;
+                }
+            }
         }
         int dstIndex = index + (numBytes - srcBytes);
         PythonUtils.arraycopy(src, srcIndex, buffer, dstIndex, srcBytes);