Fix bug in yr_rules_from_arena causing allocation of bitmask of wrong size

[1ndahous3]

There is a bug introduced in 1c309a8: the size of bitmask to allocate is calculated with undefined new_rules->num_rules value instead of passed summary->num_rules, so the YR_BITMASK_SIZE() is evaluated as random value, sometimes less then needed.

In my case new_rules->num_rules == 0 so YR_BITMASK_SIZE(new_rules->num_rules) == 1 and I get 4 (1 * sizeof(YR_BITMASK) ) bytes allocated for bitmask. After that I get heap corruption because of writing of first bit of non-allocated 5th byte for 128 rule (i == 128) here:

yara/libyara/rules.c

Lines 378 to 384 in 1c309a8

	for (int i = 0; i < new_rules->num_rules; i++)
	{
	if (new_rules->rules_table[i].required_strings == 0)
	{
	yr_bitmask_set(new_rules->rule_evaluate_condition_flags, i);
	}
	}

[1ndahous3]

@plusvic merge?