Report periodic stats only when safe to do so. [skip ci]

As explained in GH #155, when tcpdump is given -r, -w and -v and it
takes long enough to read from the input file (because it is stdin
connected through network or a pipe to stdout of another tcpdump doing
a live capture), pcap_loop() will error before long. One of the ways to
reproduce the fault is as follows:

$ tcpdump -i eno1 -w - | tcpdump -r - -w /tmp/tmp.pcap -v
tcpdump: listening on eno1, link-type EN10MB (Ethernet), snapshot length 262144 bytes
reading from file -, link-type EN10MB (Ethernet), snapshot length 262144
tcpdump: pcap_loop: error reading dump file: Interrupted system call

Skip the verbose_stats_dump() timer setup in this specific corner case
for the time being and document it.

tcpdump.1.in

@@ -20,7 +20,7 @@
 .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 .\"
-.TH TCPDUMP 1  "10 Aug 2020"
+.TH TCPDUMP 1  "29 Aug 2020"
 .SH NAME
 tcpdump \- dump traffic on a network
 .SH SYNOPSIS
@@ -808,6 +808,8 @@ IP and ICMP header checksum.
 .IP
 When writing to a file with the
 .B \-w
+option and at the same time not reading from a file with the
+.B \-r
 option, report, once per second, the number of packets captured.
 .TP
 .B \-vv

tcpdump.c

@@ -2438,11 +2438,14 @@ DIAG_ON_CLANG(assign-enum)
 	(void)setsignal(SIGNAL_FLUSH_PCAP, flushpcap);
 #endif
 
-	if (ndo->ndo_vflag > 0 && WFileName && !print) {
+	if (ndo->ndo_vflag > 0 && WFileName && RFileName == NULL && !print) {
 		/*
 		 * When capturing to a file, if "--print" wasn't specified,
 		 *"-v" means tcpdump should, once per second,
 		 * "v"erbosely report the number of packets captured.
+		 * Except when reading from a file, because -r, -w and -v
+		 * together used to make a corner case, in which pcap_loop()
+		 * errored due to EINTR (see GH #155 for details).
 		 */
 #ifdef _WIN32
 		/*