LibCpp: Fix lexing & parsing of non-terminated strings

Author: https://github.com/itamar8910
Commit: 8ace2cfa18
Pull-request: https://github.com/SerenityOS/serenity/pull/5327

Userland/Libraries/LibCpp/Lexer.cpp

@@ -640,6 +640,10 @@ Vector<Token> Lexer::lex()
                     }
                 }
 
+                // If string is not terminated - stop before EOF
+                if (!peek(1))
+                    break;
+
                 if (consume() == '"')
                     break;
             }