Fix one more missing c0_ < 0 check in scanner

src/scanner.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Features shared by parsing and pre-parsing scanners.
 
 #include <stdint.h>
 
 #include <cmath>
 
@@ skipping 294 matching lines @@
     Advance();
   }
 
   return Token::WHITESPACE;
 }
 
 
 void Scanner::TryToParseSourceURLComment() {
   // Magic comments are of the form: //[#@]\s<name>=\s*<value>\s*.* and this
   // function will just return if it cannot parse a magic comment.
-  if (!unicode_cache_->IsWhiteSpace(c0_))
-    return;
+  if (c0_ < 0 || !unicode_cache_->IsWhiteSpace(c0_)) return;
   Advance();
   LiteralBuffer name;
   while (c0_ >= 0 && !unicode_cache_->IsWhiteSpaceOrLineTerminator(c0_) &&
          c0_ != '=') {
     name.AddChar(c0_);
     Advance();
   }
   if (!name.is_one_byte()) return;
   Vector<const uint8_t> name_literal = name.one_byte_literal();
   LiteralBuffer* value;
@@ skipping 1077 matching lines @@
     }
     backing_store_.Add(static_cast<uint8_t>((one_byte_length >> 7) | 0x80u));
   }
   backing_store_.Add(static_cast<uint8_t>(one_byte_length & 0x7f));
 
   backing_store_.AddBlock(bytes);
   return backing_store_.EndSequence().start();
 }
 
 } }  // namespace v8::internal