fix ImageJ viewer NullPointerException on Snap button press

ImagePlus.getProcessor() will return null if the ImagePlus
instance has no ImageProcessor nor AWT Image.  This can occur in
EPICS_AD_Viewer.makeImageCopy(), so it needs to handle this case to
avoid a NullPointerException.

One way to reproduce this problem is to start the areaDetector
Simulator, but don't acquire any frames.  Then in the ImageJ viewer,
connect to the AD simulator Image1 plug-in (i.e. 13SIM1:image1:),
press "Start", and then press "Snap".  This will produce a
NullPointerException.  There are likely other ways to reproduce this
problem probably involving closing the image window and pressing the
"Snap" button.
Merge pull request #37 from jlmuir/fix-imagej-viewer-npe

fix ImageJ viewer NullPointerException on Snap button press