plans/objchange: Don't panic when prior state contains nested map blocks

We were using the wrong cty operation to access map members, causing a
panic whenever a prior value was present for a resource type with a nested
block backed by a map value.

plans/objchange/objchange.go

@@ -177,7 +177,7 @@ func proposedNewObject(schema *configschema.Block, prior, config cty.Value) cty.
 					atys := configV.Type().AttributeTypes()
 					for name := range atys {
 						configEV := configV.GetAttr(name)
-						if !priorV.Type().HasAttribute(name) {
+						if !priorV.IsKnown() || priorV.IsNull() || !priorV.Type().HasAttribute(name) {
 							// If there is no corresponding prior element then
 							// we just take the config value as-is.
 							newVals[name] = configEV

plans/objchange/plan_valid.go

@@ -182,7 +182,7 @@ func assertPlanValid(schema *configschema.Block, priorState, config, plannedStat
 					configEV := configV.Index(idx)
 					priorEV := cty.NullVal(blockS.ImpliedType())
 					if !priorV.IsNull() && priorV.HasIndex(idx).True() {
-						priorEV = priorV.GetAttr(k)
+						priorEV = priorV.Index(idx)
 					}
 					moreErrs := assertPlanValid(&blockS.Block, priorEV, configEV, plannedEV, path)
 					errs = append(errs, moreErrs...)

plans/objchange/plan_valid_test.go

@@ -312,6 +312,45 @@ func TestAssertPlanValid(t *testing.T) {
 			}),
 			nil,
 		},
+		"nested map, normal update": {
+			&configschema.Block{
+				BlockTypes: map[string]*configschema.NestedBlock{
+					"b": {
+						Nesting: configschema.NestingMap,
+						Block: configschema.Block{
+							Attributes: map[string]*configschema.Attribute{
+								"c": {
+									Type:     cty.String,
+									Optional: true,
+								},
+							},
+						},
+					},
+				},
+			},
+			cty.ObjectVal(map[string]cty.Value{
+				"b": cty.MapVal(map[string]cty.Value{
+					"boop": cty.ObjectVal(map[string]cty.Value{
+						"c": cty.StringVal("hello"),
+					}),
+				}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"b": cty.MapVal(map[string]cty.Value{
+					"boop": cty.ObjectVal(map[string]cty.Value{
+						"c": cty.StringVal("howdy"),
+					}),
+				}),
+			}),
+			cty.ObjectVal(map[string]cty.Value{
+				"b": cty.MapVal(map[string]cty.Value{
+					"boop": cty.ObjectVal(map[string]cty.Value{
+						"c": cty.StringVal("howdy"),
+					}),
+				}),
+			}),
+			nil,
+		},
 
 		// Nested block collections are never null
 		"nested list, null in plan": {