[SYSTEMDS-3282] Upper bound for number of decoders

This patch adds a coherence check on the number of deserialized
decoders in DecoderComposite object.

Closes #1527.

Author: phaniarnab

src/main/java/org/apache/sysds/runtime/transform/decode/DecoderComposite.java

@@ -83,6 +83,7 @@ public void initMetaData(FrameBlock meta) {
 	public void writeExternal(ObjectOutput out) throws IOException {
 		super.writeExternal(out);
 		out.writeInt(_decoders.size());
+		out.writeInt(_schema == null ? 0:_schema.length); //write #columns
 		for(Decoder decoder : _decoders) {
 			out.writeByte(DecoderFactory.getDecoderType(decoder));
 			decoder.writeExternal(out);
@@ -93,6 +94,9 @@ public void writeExternal(ObjectOutput out) throws IOException {
 	public void readExternal(ObjectInput in) throws IOException {
 		super.readExternal(in);
 		int decodersSize = in.readInt();
+		int nCols = in.readInt();
+		if (nCols > 0 && decodersSize > nCols*2)
+			throw new IOException("Too many decoders");
 		_decoders = new ArrayList<>();
 		for(int i = 0; i < decodersSize; i++) {
 			Decoder decoder = DecoderFactory.createInstance(in.readByte());