Check deck sizes on startup in case they were only partly built

closes #31

src/SnowMerkleProof.java

@@ -116,6 +116,12 @@ public SnowMerkleProof(File path, String base)
 			RandomAccessFile deck_file = new RandomAccessFile(new File(path, base +".deck." + letter), "r");
 			FileChannel deck_channel = deck_file.getChannel();
 
+      long expected_len = snow_file.length() / h;
+      if (deck_file.length() != expected_len)
+      {
+        throw new java.io.IOException("Unexpected length on " + base +".deck." + letter);
+      }
+
 			deck_map.put(h, deck_channel);
 		}
 		deck_files = ImmutableMap.copyOf(deck_map);

src/miner/FieldScan.java

@@ -9,7 +9,7 @@
 import java.io.File;
 
 import java.util.logging.Logger;
-import java.util.Random;
+import java.util.SplittableRandom;
 
 import java.util.Map;
 import java.util.SortedMap;
@@ -87,13 +87,11 @@ public void scan(boolean report)
   private void checkField(int field_number, SnowMerkleProof proof)
     throws java.io.IOException
   {
-    Random rnd = new Random();
+    SplittableRandom rnd = new SplittableRandom();
 
-    // If the field ends up with more than 32 bits of words, just check one from
-    // the first part. meh.
-    long max = Math.min((long)Integer.MAX_VALUE, proof.getTotalWords());
+    long max = proof.getTotalWords();
 
-    long check = rnd.nextInt((int)max);
+    long check = rnd.nextLong(max);
     SnowPowProof p = proof.getProof(check);
 
     SnowFieldInfo info = params.getSnowFieldInfo(field_number);