roken: strtoull.c negation is a no-op on unsigned integer

strtoull() returns an unsigned long long.  However, then the input
string represents a negative number the return value is supposed to
be the unsigned representation of the negative value.  Before applying
the negation the value must be cast to (long long).

Change-Id: Icf9e75400ff736819b1f7e0e6fb3c8abd707a23a

lib/roken/strtoull.c

@@ -119,9 +119,8 @@ strtoull(const char * nptr, char ** endptr, int base)
 noconv:
 	errno = EINVAL;
     } else if (neg)
-	acc = -acc;
+	acc = -(long long)acc;
     if (endptr != NULL)
 	*endptr = (char *)(any ? s - 1 : nptr);
     return (acc);
 }
-