[spotbugs] Fix FileReftableStack#equals to check for null

This fixes spotbugs warning NP_EQUALS_SHOULD_HANDLE_NULL_ARGUMENT.

This implementation violated the contract defined by
java.lang.Object.equals() because it did not check for null being passed
as the argument. All equals() methods should return false if passed a
null value.

Change-Id: I607f6979613d390aae2f3546b587f63133d6d73c
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileReftableStack.java

@@ -636,6 +636,9 @@ public class FileReftableStack implements AutoCloseable {
 
 		@Override
 		public boolean equals(Object other) {
+			if (other == null) {
+				return false;
+			}
 			Segment o = (Segment) other;
 			return o.bytes == bytes && o.log == log && o.start == start
 					&& o.end == end;