Avoid unnecessary cast to 64-bit

Fixes https://github.com/behdad/harfbuzz/issues/146

Or I think it should.

src/hb-private.hh

@@ -242,7 +242,7 @@ static inline unsigned int ARRAY_LENGTH (const Type (&)[n]) { return n; }
 
 /* Note: C++ allows sizeof() of variable-lengh arrays.  So, if _cond is not
  * constant, it still compiles (ouch!), but at least we'll get a -Wvla warning. */
-#define ASSERT_STATIC_EXPR_ZERO(_cond) (0 * sizeof (char[(_cond) ? 1 : -1]))
+#define ASSERT_STATIC_EXPR_ZERO(_cond) (0 * (unsigned int) sizeof (char[(_cond) ? 1 : -1]))
 
 #define _PASTE1(a,b) a##b
 #define PASTE(a,b) _PASTE1(a,b)