Fix an hang on 32 bits while allocating big TT table

If size_t is defined as a 32 bit quanitity then we have an
overflow in the left term of the while condition if mbSize
is bigger then 2048.

For instance if mbSize is 2049 then when newSize will reach
0x80000000 (2048MB) comparison is still true, 'while' loops
again and we have an overflow in the expression (2*newSize)
so that result is 0 and at that point 'while' keeps looping
forever hanging the application.

This patch fixes the bug and also makes operator new do not
throw an exception upon failure but return a NULL pointer
instead.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/tt.h

@@ -46,7 +46,7 @@ class SimpleHash {
 public:
   SimpleHash() {
 
-    entries = new Entry[HashSize];
+    entries = new (std::nothrow) Entry[HashSize];
     if (!entries)
     {
         std::cerr << "Failed to allocate " << HashSize * sizeof(Entry)