Fix null pointer dereference in status.ByIndex (#628)

`git_status_byindex` can return a null pointer if there is no statuses.

status.go

@@ -6,6 +6,7 @@ package git
 import "C"
 
 import (
+	"errors"
 	"runtime"
 	"unsafe"
 )
@@ -86,6 +87,9 @@ func (statusList *StatusList) ByIndex(index int) (StatusEntry, error) {
 		return StatusEntry{}, ErrInvalid
 	}
 	ptr := C.git_status_byindex(statusList.ptr, C.size_t(index))
+	if ptr == nil {
+		return StatusEntry{}, errors.New("index out of Bounds")
+	}
 	entry := statusEntryFromC(ptr)
 	runtime.KeepAlive(statusList)
 

status_test.go

@@ -61,3 +61,31 @@ func TestStatusList(t *testing.T) {
 		t.Fatal("Incorrect entry path: ", entry.IndexToWorkdir.NewFile.Path)
 	}
 }
+
+func TestStatusNothing(t *testing.T) {
+	t.Parallel()
+	repo := createTestRepo(t)
+	defer cleanupTestRepo(t, repo)
+
+	seedTestRepo(t, repo)
+
+	opts := &StatusOptions{
+		Show:  StatusShowIndexAndWorkdir,
+		Flags: StatusOptIncludeUntracked | StatusOptRenamesHeadToIndex | StatusOptSortCaseSensitively,
+	}
+
+	statusList, err := repo.StatusList(opts)
+	checkFatal(t, err)
+
+	entryCount, err := statusList.EntryCount()
+	checkFatal(t, err)
+
+	if entryCount != 0 {
+		t.Fatal("expected no statuses in empty repo")
+	}
+
+	_, err = statusList.ByIndex(0)
+	if err == nil {
+		t.Error("expected error getting status by index")
+	}
+}