Don't crash when assigning group array to array

parser.go

@@ -107,7 +107,7 @@ func (p *tomlParser) parseGroupArray() tomlParserStateFn {
 	var array []*TomlTree
 	if destTree == nil {
 		array = make([]*TomlTree, 0)
-	} else if destTree.([]*TomlTree) != nil {
+	} else if target, ok := destTree.([]*TomlTree); ok && target != nil {
 		array = destTree.([]*TomlTree)
 	} else {
 		p.raiseError(key, "key %s is already assigned and not of type group array", key)

parser_test.go

@@ -626,3 +626,10 @@ func TestDoubleEqual(t *testing.T) {
 		t.Error("Bad error message:", err.Error())
 	}
 }
+
+func TestGroupArrayReassign(t *testing.T) {
+	_, err := Load("[hello]\n[[hello]]")
+	if err.Error() != "(2, 3): key \"hello\" is already assigned and not of type group array" {
+		t.Error("Bad error message:", err.Error())
+	}
+}