Fix some crashes in GN.

GN would read past the end of the file if the last line was a comment that didn't end in a newline.

It also would get confused if an end-of-file was reached after an assignment or a unary operator.

BUG=478169

Review URL: https://codereview.chromium.org/1275853006

Cr-Commit-Position: refs/heads/master@{#342417}

tools/gn/parser.cc

@@ -374,6 +374,11 @@ scoped_ptr<ParseNode> Parser::Not(Token token) {
   scoped_ptr<ParseNode> expr = ParseExpression(PRECEDENCE_PREFIX + 1);
   if (has_error())
     return scoped_ptr<ParseNode>();
+  if (!expr) {
+    if (!has_error())
+      *err_ = Err(token, "Expected right-hand side for '!'.");
+    return scoped_ptr<ParseNode>();
+  }
   scoped_ptr<UnaryOpNode> unary_op(new UnaryOpNode);
   unary_op->set_op(token);
   unary_op->set_operand(expr.Pass());
@@ -393,7 +398,7 @@ scoped_ptr<ParseNode> Parser::BinaryOperator(scoped_ptr<ParseNode> left,
       ParseExpression(expressions_[token.type()].precedence + 1);
   if (!right) {
     if (!has_error()) {
-      *err_ = Err(token, "Expected right hand side for '" +
+      *err_ = Err(token, "Expected right-hand side for '" +
                              token.value().as_string() + "'");
     }
     return scoped_ptr<ParseNode>();
@@ -451,6 +456,11 @@ scoped_ptr<ParseNode> Parser::Assignment(scoped_ptr<ParseNode> left,
     return scoped_ptr<ParseNode>();
   }
   scoped_ptr<ParseNode> value = ParseExpression(PRECEDENCE_ASSIGNMENT);
+  if (!value) {
+    if (!has_error())
+      *err_ = Err(token, "Expected right-hand side for assignment.");
+    return scoped_ptr<ParseNode>();
+  }
   scoped_ptr<BinaryOpNode> assign(new BinaryOpNode);
   assign->set_op(token);
   assign->set_left(left.Pass());

tools/gn/parser_unittest.cc

@@ -181,6 +181,9 @@ TEST(Parser, UnaryOp) {
   DoExpressionPrintTest("!foo",
       "UNARY(!)\n"
       " IDENTIFIER(foo)\n");
+
+  // No contents for binary operator.
+  DoExpressionErrorTest("a = !", 1, 5);
 }
 
 TEST(Parser, List) {
@@ -215,6 +218,8 @@ TEST(Parser, Assignment) {
                     " BINARY(=)\n"
                     "  IDENTIFIER(a)\n"
                     "  LITERAL(2)\n");
+
+  DoExpressionErrorTest("a = ", 1, 3);
 }
 
 TEST(Parser, Accessor) {

tools/gn/tokenizer.cc

@@ -128,6 +128,7 @@ std::vector<Token> Tokenizer::Run() {
                location.line_number() ||
            tokens_.back().location().char_offset() != location.char_offset())) {
         type = Token::LINE_COMMENT;
+        if (!at_end())  // Could be EOF.
           Advance();  // The current \n.
         // If this comment is separated from the next syntax element, then we
         // want to tag it as a block comment. This will become a standalone