Fix panic on non-string topic name

A panic results in an unhelpful message when running the plugin:

    Failed to check ruleset; error reading from server: EOF

So report an error here instead.

There is potential for similar behaviour if `consume_topics` is not a
list, in which case `AsValueSlice` would panic, but I'm content to not
be too defensive here if no-ones needing that change.

Ticket: DENA-1175

rules/msk_app_topics.go

@@ -153,6 +153,22 @@ func (r *MSKAppTopicsRule) reportExternalTopics(
 		return fmt.Errorf("evaluating topic names: %w", diags)
 	}
 	for _, v := range val.AsValueSlice() {
+		if v.Type() != cty.String {
+			err := runner.EmitIssue(
+				r,
+				fmt.Sprintf(
+					"value for '%s' must be a string, not: %s",
+					attrName,
+					v.Type().FriendlyName(),
+				),
+				topicAttr.Range,
+			)
+			if err != nil {
+				return fmt.Errorf("emitting issue: %w", err)
+			}
+			continue
+		}
+
 		name := v.AsString()
 		if _, ok := moduleTopicNames[name]; !ok {
 			err := runner.EmitIssue(

rules/msk_app_topics_test.go

@@ -89,6 +89,31 @@ module "consumer" {
 				},
 			},
 		},
+		{
+			name: "topic name is not string",
+			files: map[string]string{
+				"file.tf": `
+resource "kafka_topic" "my_topic" {
+	name = "my_topic"
+}
+
+module "consumer" {
+	consume_topics = [kafka_topic.my_topic]
+}
+`,
+			},
+			expected: []*helper.Issue{
+				{
+					Rule:    rule,
+					Message: "value for 'consume_topics' must be a string, not: object",
+					Range: hcl.Range{
+						Filename: "file.tf",
+						Start:    hcl.Pos{Line: 7, Column: 2},
+						End:      hcl.Pos{Line: 7, Column: 41},
+					},
+				},
+			},
+		},
 		{
 			name: "external topic defined outside of consumer/producer",
 			files: map[string]string{