MP_FREE_DIGITS: add missing cast

Author: minad

tommath_private.h

@@ -31,7 +31,7 @@ extern "C" {
  */
 #ifdef MP_NO_ZERO_ON_FREE
 #  define MP_FREE_BUFFER(mem, size)   MP_FREE((mem), (size))
-#  define MP_FREE_DIGITS(mem, digits) MP_FREE((mem), sizeof (mp_digit) * (digits))
+#  define MP_FREE_DIGITS(mem, digits) MP_FREE((mem), sizeof (mp_digit) * (size_t)(digits))
 #else
 #  define MP_FREE_BUFFER(mem, size)                     \
 do {                                                    \