Fix the implementation of StreamingXXHash32.asChecksum()

In the Checksum view, we only keep the last 28 bits of the hash instead
of the 32 bits of the full hash.

The result is that the 4 first bits are always set to 0 in
LZ4Block*Stream.

This commit:
- fixes the Checksum view in StreamingXXHash32
- keeps the current behaviour in LZ4Block*Stream to avoid breaking
  existing data built with this library

src/java/net/jpountz/lz4/LZ4BlockInputStream.java

@@ -257,7 +257,7 @@ private void refill() throws IOException {
     }
     checksum.reset();
     checksum.update(buffer, 0, originalLen);
-    if ((int) checksum.getValue() != check) {
+    if ((int) (checksum.getValue() & 0x0FFFFFFF) != check) {
       throw new IOException("Stream is corrupted");
     }
     o = 0;

src/java/net/jpountz/lz4/LZ4BlockOutputStream.java

@@ -206,7 +206,7 @@ private void flushBufferedData() throws IOException {
     }
     checksum.reset();
     checksum.update(buffer, 0, o);
-    final int check = (int) checksum.getValue();
+    final int check = (int) (checksum.getValue() & 0x0FFFFFFF);
     int compressedLength = compressor.compress(buffer, 0, o, compressedBuffer, HEADER_LENGTH);
     final int compressMethod;
     if (compressedLength >= o) {

src/java/net/jpountz/xxhash/StreamingXXHash32.java

@@ -103,7 +103,7 @@ public final Checksum asChecksum() {
 
       @Override
       public long getValue() {
-        return StreamingXXHash32.this.getValue() & 0xFFFFFFFL;
+        return StreamingXXHash32.this.getValue() & 0xFFFFFFFFL;
       }
 
       @Override