index: fix NULL pointer access in index_remove_entry

When removing an entry from the index by its position, we first
retrieve the position from the index's entries and then try to
remove the retrieved value from the index map with
`DELETE_IN_MAP`. When `index_remove_entry` returns `NULL` we try
to feed it into the `DELETE_IN_MAP` macro, which will
unconditionally call `idxentry_hash` and then happily dereference
the `NULL` entry pointer.

Fix the issue by not passing a `NULL` entry into `DELETE_IN_MAP`.

src/index.c

@@ -505,10 +505,11 @@ static int index_remove_entry(git_index *index, size_t pos)
 	int error = 0;
 	git_index_entry *entry = git_vector_get(&index->entries, pos);
 
-	if (entry != NULL)
+	if (entry != NULL) {
 		git_tree_cache_invalidate_path(index->tree, entry->path);
+		DELETE_IN_MAP(index, entry);
+	}
 
-	DELETE_IN_MAP(index, entry);
 	error = git_vector_remove(&index->entries, pos);
 
 	if (!error) {