Use word arithmetic for SegmentReader

Prior to this change, the start index was treated as a byte index
rather than a word index.

runtime/src/main/java/org/capnproto/SegmentReader.java

@@ -43,9 +43,10 @@ public final long get(int index) {
      * Verify that the `size`-long (in words) range starting at word index
      * `start` is within bounds.
      */
-    public final boolean isInBounds(int start, int size) {
-        if (start < 0 || size < 0) return false;
-        long sizeInWords = (long) size * Constants.BYTES_PER_WORD;
-        return start + sizeInWords <= this.buffer.capacity();
+    public final boolean isInBounds(int startInWords, int sizeInWords) {
+        if (startInWords < 0 || sizeInWords < 0) return false;
+        long startInBytes = (long) startInWords * Constants.BYTES_PER_WORD;
+        long sizeInBytes = (long) sizeInWords * Constants.BYTES_PER_WORD;
+        return startInBytes + sizeInBytes <= this.buffer.capacity();
     }
 }

runtime/src/test/java/org/capnproto/SegmentReaderTest.java

@@ -15,4 +15,18 @@ public void in_boundsCalculationShouldNotOverflow() {
         SegmentReader segmentReader = new SegmentReader(byteBuffer, null);
         MatcherAssert.assertThat(segmentReader.isInBounds(0, Integer.MAX_VALUE), is(false));
     }
+
+    @Test
+    public void oneWordAtLastWordShouldBeInBounds() {
+        ByteBuffer byteBuffer = ByteBuffer.allocate(64);
+        SegmentReader segmentReader = new SegmentReader(byteBuffer, null);
+        MatcherAssert.assertThat(segmentReader.isInBounds(7, 1), is(true));
+    }
+
+    @Test
+    public void twoWordsAtLastWordShouldNotBeInBounds() {
+        ByteBuffer byteBuffer = ByteBuffer.allocate(64);
+        SegmentReader segmentReader = new SegmentReader(byteBuffer, null);
+        MatcherAssert.assertThat(segmentReader.isInBounds(7, 2), is(false));
+    }
 }