postgresql: Fix panic walking CreateTableAsStmt (#475)

Author: kyleconroy

internal/endtoend/testdata/materialized_views/go/db.go

@@ -0,0 +1,11 @@
+CREATE TABLE authors (
+    id   BIGSERIAL PRIMARY KEY,
+    name TEXT NOT NULL,
+    bio  TEXT
+);
+
+ALTER TABLE authors ADD COLUMN gender INTEGER NULL;
+
+CREATE MATERIALIZED VIEW authors_names as SELECT name from authors;
+
+SELECT * FROM authors;

internal/endtoend/testdata/materialized_views/go/models.go

@@ -0,0 +1,9 @@
+{
+  "version": "1",
+  "packages": [{
+    "path": "go",
+	"name": "querytest",
+	"schema": "query.sql",
+	"queries": "query.sql"
+  }]
+}

internal/endtoend/testdata/materialized_views/query.sql

@@ -496,7 +496,9 @@ func Walk(f Visitor, node nodes.Node) {
 
 	case nodes.CreateTableAsStmt:
 		walkn(f, n.Query)
-		walkn(f, n.Into)
+		if n.Into != nil {
+			walkn(f, *n.Into)
+		}
 
 	case nodes.CreateTableSpaceStmt:
 		if n.Owner != nil {