Use sizeof(LONG_LONG) instead of magic number

Author: nurse

ext/mysql2/statement.c

@@ -218,7 +218,7 @@ static int my_big2ll(VALUE bignum, LONG_LONG *ptr)
 #else
   len = RBIGNUM_LEN(bignum) * SIZEOF_BDIGITS;
 #endif
-  if (len > 8) goto overflow;
+  if (len > sizeof(LONG_LONG)) goto overflow;
   if (RBIGNUM_POSITIVE_P(bignum)) {
     num = rb_big2ull(bignum);
     if (num > LLONG_MAX)