AVRO-3635: Disallow skipping a negative amount of bytes (#2997)

This is what all other implementations of this method do, and fixes
infinite loops due to malicious data.

lang/java/avro/src/main/java/org/apache/avro/io/BinaryDecoder.java

@@ -17,17 +17,17 @@
  */
 package org.apache.avro.io;
 
+import org.apache.avro.AvroRuntimeException;
+import org.apache.avro.InvalidNumberEncodingException;
+import org.apache.avro.SystemLimitException;
+import org.apache.avro.util.Utf8;
+
 import java.io.EOFException;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 
-import org.apache.avro.AvroRuntimeException;
-import org.apache.avro.InvalidNumberEncodingException;
-import org.apache.avro.SystemLimitException;
-import org.apache.avro.util.Utf8;
-
 /**
  * An {@link Decoder} for binary-format data.
  * <p/>
@@ -338,6 +338,9 @@ public int readEnum() throws IOException {
   }
 
   protected void doSkipBytes(long length) throws IOException {
+    if (length <= 0) {
+      return;
+    }
     int remaining = limit - pos;
     if (length <= remaining) {
       pos = (int) (pos + length);

lang/java/avro/src/test/java/org/apache/avro/io/TestBinaryDecoder.java

@@ -661,6 +661,7 @@ private void skipGenerated(Decoder bd) throws IOException {
       // booleans are one byte, array trailer is one byte
       bd.skipFixed((int) leftover + 1);
       bd.skipFixed(0);
+      bd.skipFixed(-8); // Should be a no-op; see AVRO-3635
       bd.readLong();
     }
     EOFException eof = null;