Manifest: add some defensive programming

This guards against the wrapped manifest being null, as well as the
wrapped manifest's main attributes being null for some reason.

Intended to address (the symptom, at least) of Fiji bug 889:

    http://fiji.sc/bugzilla/show_bug.cgi?id=889

src/main/java/org/scijava/util/Manifest.java

@@ -36,6 +36,7 @@
 import java.net.URL;
 import java.util.Collections;
 import java.util.Map;
+import java.util.jar.Attributes;
 
 /**
  * Helper class for working with JAR manifests.
@@ -115,11 +116,17 @@ public String getSpecificationVersion() {
 	}
 
 	public String get(final String key) {
-		return manifest.getMainAttributes().getValue(key);
+		if (manifest == null) return null;
+		final Attributes mainAttrs = manifest.getMainAttributes();
+		if (mainAttrs == null) return null;
+		return mainAttrs.getValue(key);
 	}
 
 	public Map<Object, Object> getAll() {
-		return Collections.unmodifiableMap(manifest.getMainAttributes());
+		if (manifest == null) return null;
+		final Attributes mainAttrs = manifest.getMainAttributes();
+		if (mainAttrs == null) return null;
+		return Collections.unmodifiableMap(mainAttrs);
 	}
 
 	// -- Utility methods --