Stop DSLParser from continuing after an invalid token

This matches the behavior of SkSLParser

Change-Id: Ib9af7173dda8866faf8633b8c88a8716b4e6b8d4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/443239
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>

src/sksl/SkSLDSLParser.cpp

@@ -225,6 +225,7 @@ std::unique_ptr<Program> DSLParser::program() {
             case Token::Kind::TK_INVALID: {
                 this->nextToken();
                 this->error(this->peek(), String("invalid token"));
+                done = true;
                 break;
             }
             default: