Defensive programming in DefaultLegacyOpener

When loading an image via a URL and calling File>Revert, the module
returned by the module service can be null... Symptom:

java.lang.NullPointerException
	at net.imagej.legacy.plugin.DefaultLegacyOpener.open(DefaultLegacyOpener.java:116)
	at net.imagej.legacy.DefaultLegacyHooks.interceptOpen(DefaultLegacyHooks.java:400)
	at ij.IJ.openImage(IJ.java)
	at ij.io.FileOpener.revertToSaved(FileOpener.java:258)
	at ij.ImagePlus.revert(ImagePlus.java:1719)
	at ij.plugin.Commands.revert(Commands.java:53)
	at ij.plugin.Commands.run(Commands.java:35)
	at ij.IJ.runPlugIn(IJ.java:171)
	at ij.Executer.runCommand(Executer.java:131)
	at ij.Executer.run(Executer.java:64)
	at java.lang.Thread.run(Thread.java:701)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #19 from imagej/legacy-opener

Defensive programming in DefaultLegacyOpener