pcap_findalldevs(3PCAP): *alldevsp is set to NULL on error. [skip ci]

I.e., if pcap_findalldevs() fails, there is no partically-constructed
list that its caller needs to free.

Ping #1211; this fixes the documentation to match the way the code
has worked insce pcap_findalldevs() was added as an API.

pcap_findalldevs.3pcap

@@ -238,7 +238,11 @@ returned in that case. If
 .B PCAP_ERROR
 is returned,
 .I errbuf
-is filled in with an appropriate error message.
+is filled in with an appropriate error message,
+and the pointer pointed to by
+.I alldevsp
+is set to
+.BR NULL .
 .SH BACKWARD COMPATIBILITY
 .PP
 The