Fix issues found by Coverity

Throw an exception if the file stream is shorter than expected.

Make inner result class static.

Author: oschwald

src/main/java/com/maxmind/db/BufferHolder.java

@@ -22,7 +22,11 @@ final class BufferHolder {
             final FileChannel channel = file.getChannel();
             if (mode == FileMode.MEMORY) {
                 this.buffer = ByteBuffer.wrap(new byte[(int) channel.size()]);
-                channel.read(this.buffer);
+                if (channel.read(this.buffer) != this.buffer.capacity()) {
+                    throw new IOException("Unable to read "
+                            + database.getName()
+                            + " into memory. Unexpected end of stream.");
+                }
             } else {
                 this.buffer = channel.map(MapMode.READ_ONLY, 0, channel.size());
             }

src/main/java/com/maxmind/db/Decoder.java

@@ -7,16 +7,7 @@
 
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
-import com.fasterxml.jackson.databind.node.ArrayNode;
-import com.fasterxml.jackson.databind.node.BigIntegerNode;
-import com.fasterxml.jackson.databind.node.BinaryNode;
-import com.fasterxml.jackson.databind.node.BooleanNode;
-import com.fasterxml.jackson.databind.node.DoubleNode;
-import com.fasterxml.jackson.databind.node.FloatNode;
-import com.fasterxml.jackson.databind.node.IntNode;
-import com.fasterxml.jackson.databind.node.LongNode;
-import com.fasterxml.jackson.databind.node.ObjectNode;
-import com.fasterxml.jackson.databind.node.TextNode;
+import com.fasterxml.jackson.databind.node.*;
 
 /*
  * Decoder for MaxMind DB data.
@@ -55,7 +46,7 @@ public static Type fromControlByte(int b) {
         }
     }
 
-    class Result {
+    static class Result {
         private final JsonNode node;
         private int offset;