Fix dig hanging issue in cases when the lookup's next query can't start

In recv_done(), when dig decides to start the lookup's next query in
the line using `start_udp()` or `start_tcp()`, and for some reason,
no queries get started, dig doesn't cancel the lookup.

This can occur, for example, when there are two queries in the lookup,
one with a regular IP address, and another with a IPv4 mapped IPv6
address. When the regular IP address fails to serve the query, its
`recv_done()` callback starts the next query in the line (in this
case the one with a mapped IP address), but because `dig` doesn't
connect to such IP addresses, and there are no other queries in the
list, no new queries are being started, and the lookup keeps hanging.

After calling `start_udp()` or `start_tcp()` in `recv_done()`, check
if there are no pending/working queries then cancel the lookup instead
of only detaching from the current query.

(cherry picked from commit 7e2f50c369)

bin/tests/system/digdelv/tests.sh

@@ -1071,6 +1071,16 @@ if [ -x "$DIG" ] ; then
   grep "status: NOERROR" dig.out.test$n > /dev/null || ret=1
   if [ $ret -ne 0 ]; then echo_i "failed"; fi
   status=$((status+ret))
+
+  # See [GL #3248] for more information
+  n=$((n+1))
+  echo_i "check that dig correctly refuses to use a server with a IPv4 mapped IPv6 address after failing with a regular IP address ($n)"
+  ret=0
+  dig_with_opts @10.53.0.8 @::ffff:10.53.0.8 a.example > dig.out.test$n 2>&1 || ret=1
+  grep -F ";; Skipping mapped address" dig.out.test$n > /dev/null || ret=1
+  grep -F ";; No acceptable nameservers" dig.out.test$n > /dev/null || ret=1
+  if [ $ret -ne 0 ]; then echo_i "failed"; fi
+  status=$((status+ret))
 else
   echo_i "$DIG is needed, so skipping these dig tests"
 fi