DataHandle: return length -1 when non-existent

This avoids a problem where asking a FileHandle for its
length causes a previously non-existent file to be created.

Author: ctrueden

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

@@ -123,7 +123,11 @@ default String checksum() throws IOException {
 	 */
 	void seek(long pos) throws IOException;
 
-	/** Returns the length of the data in bytes. */
+	/**
+	 * Returns the length of the data in bytes.
+	 * 
+	 * @return The length, or -1 if the length is unknown.
+	 */
 	long length() throws IOException;
 
 	/**

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

@@ -103,7 +103,7 @@ public long offset() throws IOException {
 
 	@Override
 	public long length() throws IOException {
-		return raf().length();
+		return exists() ? raf().length() : -1;
 	}
 
 	@Override