fixes alignment issue in waveform canvas

plugins/com.minres.scviewer.database.ui.swt/src/com/minres/scviewer/database/ui/swt/internal/WaveformCanvas.java

@@ -363,6 +363,9 @@ public class WaveformCanvas extends Canvas implements IWaveformZoom{
 		} else { /* image is less higher than client area */
 			vertical.setMaximum(clientHeight);
 			vertical.setEnabled(false);
+			if ( -origin.y > vertical.getMaximum() - height) {
+				origin.y = -vertical.getMaximum() + height;
+			}
 		}
 		vertical.setThumb(clientHeight);
 		vertical.setSelection(-origin.y);