provider/template: template_dir: don't crash if source dir nonexistent

When an error is passed, the FileInfo can be nil, which was previously
causing a crash on trying to evaluate f.IsDir(). By checking for an error
first we avoid this crash.

builtin/providers/template/resource_template_dir.go

@@ -89,12 +89,12 @@ func resourceTemplateDirCreate(d *schema.ResourceData, meta interface{}) error {
 
 	// Recursively crawl the input files/directories and generate the output ones.
 	err := filepath.Walk(sourceDir, func(p string, f os.FileInfo, err error) error {
-		if f.IsDir() {
-			return nil
-		}
 		if err != nil {
 			return err
 		}
+		if f.IsDir() {
+			return nil
+		}
 
 		relPath, _ := filepath.Rel(sourceDir, p)
 		return generateDirFile(p, path.Join(destinationDir, relPath), f, vars)