An empty module in state can panic

An empty module, or a module with an empty path can panic during graph
traversal. Make sure we clear these out when reading and writing the
state.

terraform/state.go

@@ -644,6 +644,17 @@ func (s *State) init() {
 	}
 	s.ensureHasLineage()
 
+	// Filter out empty modules.
+	// A module is always assumed to have a path, and it's length isn't always
+	// bounds checked later on. Modules may be "emptied" during destroy, but we
+	// never want to store those in the state.
+	for i := 0; i < len(s.Modules); i++ {
+		if s.Modules[i] == nil || len(s.Modules[i].Path) == 0 {
+			s.Modules = append(s.Modules[:i], s.Modules[i+1:]...)
+			i--
+		}
+	}
+
 	for _, mod := range s.Modules {
 		mod.init()
 	}

terraform/state_test.go

@@ -1674,3 +1674,37 @@ func TestParseResourceStateKey(t *testing.T) {
 		}
 	}
 }
+
+func TestStateModuleOrphans_empty(t *testing.T) {
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: RootModulePath,
+			},
+			&ModuleState{
+				Path: []string{RootModuleName, "foo", "bar"},
+			},
+			&ModuleState{
+				Path: []string{},
+			},
+			nil,
+		},
+	}
+
+	state.init()
+
+	// just calling this to check for panic
+	state.ModuleOrphans(RootModulePath, nil)
+
+	for _, mod := range state.Modules {
+		if mod == nil {
+			t.Fatal("found nil module")
+		}
+		if mod.Path == nil {
+			t.Fatal("found nil module path")
+		}
+		if len(mod.Path) == 0 {
+			t.Fatal("found empty module path")
+		}
+	}
+}