Add a check to decide whether use writer() or reader()

Author: ktian8

src/main/java/org/scijava/io/handle/FileHandle.java

@@ -136,7 +136,12 @@ public int read(final byte[] b, final int off, final int len)
 
 	@Override
 	public void seek(final long pos) throws IOException {
-		reader().seek(pos);
+		if (isWritable()) {
+			writer().seek(pos);
+		}
+		else {
+			reader().seek(pos);
+		}
 	}
 
 	// -- DataInput methods --