Fix read of uninitialized data on invalid input in the first token.

PiperOrigin-RevId: 561028957

src/google/protobuf/compiler/parser_unittest.cc

@@ -1134,6 +1134,19 @@ TEST_F(ParseMessageTest, ExplicitOptionalLabelProto3) {
       "}");
 }
 
+TEST_F(ParseMessageTest, CanHandleErrorOnFirstToken) {
+  require_syntax_identifier_ = false;
+  ExpectHasEarlyExitErrors(
+      "/", "0:0: Expected top-level statement (e.g. \"message\").\n");
+
+  require_syntax_identifier_ = true;
+  ExpectHasEarlyExitErrors(
+      "/",
+      "0:0: Expected top-level statement (e.g. \"message\").\n"
+      "0:0: File must begin with a syntax statement, e.g. 'syntax = "
+      "\"proto2\";'.\n");
+}
+
 // ===================================================================
 
 typedef ParserTest ParseEnumTest;

src/google/protobuf/io/tokenizer.cc

@@ -235,6 +235,7 @@ Tokenizer::Tokenizer(ZeroCopyInputStream* input,
   current_.column = 0;
   current_.end_column = 0;
   current_.type = TYPE_START;
+  previous_ = current_;
 
   Refresh();
 }