Correctly zero-initialize MainThread

Correctly zero-initialize MainThread

It can be used uninitialized in time management.
Fixes all valgrind errors on './stockfish go wtime 8000 btime 8000 winc 500 binc 500'

This is one (of the many) quirks of C++. There is a subtle difference between:

new Foo
new Foo()

The first statement calls the default constructor (that in case of a POD leaves data members
uninitialized), the second one performs a value-initialization (that in case of POD is
equivalent to a zero-initialization)

See:
http://stackoverflow.com/questions/620137/do-the-parentheses-after-the-type-name-make-a-difference-with-new
http://stackoverflow.com/questions/5116541/difference-between-creating-object-with-or-without

No functional change.