Fix issue 10398

http://d.puremagic.com/issues/show_bug.cgi?id=10398

These changes fix some memory issues in dmd detected by memcheck during druntime and phobos compilation.

1. There are issues with local uninitialized variable code cs; which is later used in compare expressions with undefined value.

2. in Lexer::nextToken() *t pointer is uninitialized and its value is later copied.

3. Code which indirectly calls StringEntry::alloc may exhibit off-by-one error.