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.

src/thread.cpp

@@ -123,7 +123,7 @@ void Thread::idle_loop() {
 
 void ThreadPool::init() {
 
-  push_back(new MainThread);
+  push_back(new MainThread());
   read_uci_options();
 }
 
@@ -150,7 +150,7 @@ void ThreadPool::read_uci_options() {
   assert(requested > 0);
 
   while (size() < requested)
-      push_back(new Thread);
+      push_back(new Thread());
 
   while (size() > requested)
       delete back(), pop_back();