Remote packet capture support is disabled by default in libpcap

Remote packet capture support is disabled by default in libpcap. When a user builds libpcap with remote packet capture support enabled, one of the functions that become available is pcap_findalldevs_ex(). One of the function arguments can be a filesystem path, which normally means a directory with input data files. When the specified path cannot be used as a directory, the function receives NULL from opendir(), but does not check the return value and passes the NULL value to readdir(), which causes a NULL pointer derefence.