Allow deletions to occur when there is no HEAD

If the HEAD is not present in a repository, then there is a
NullPointerException thrown in the delete code. Since this only
exists to verify if the deletion is not the HEAD reference, then
skip this check if the HEAD cannot be found.

Bug: 406722
Change-Id: I882497202d986096513a4d791cd07fa935a3f9e4
Signed-off-by: Alex Blewitt <alex.blewitt@gmail.com>

org.eclipse.jgit/src/org/eclipse/jgit/lib/RefUpdate.java

@@ -531,7 +531,7 @@ public abstract class RefUpdate {
 		final String myName = getRef().getLeaf().getName();
 		if (myName.startsWith(Constants.R_HEADS)) {
 			Ref head = getRefDatabase().getRef(Constants.HEAD);
-			while (head.isSymbolic()) {
+			while (head != null && head.isSymbolic()) {
 				head = head.getTarget();
 				if (myName.equals(head.getName()))
 					return result = Result.REJECTED_CURRENT_BRANCH;