Description of problem:
Just noticed this, dunno about situations where it actually happened.
I think I've seen the abort previously but I have not reported them.

Version-Release number of selected component:
systemd-239-6.git9f3aed1.fc29

Additional info:
reporter:       libreport-2.9.7
backtrace_rating: 4
cmdline:        /usr/lib/systemd/systemd-resolved
crash_function: log_assert_failed_realm
executable:     /usr/lib/systemd/systemd-resolved
journald_cursor: s=011a87a772524b04ab4912ae8f8e64ea;i=17b963;b=02ea55a5838c4bb3a92617f3b362078c;m=18030d880e;t=57c749070b3f6;x=d8843d214c140c5b
kernel:         4.19.6-300.fc29.x86_64
rootdir:        /
runlevel:       N 5
type:           CCpp
uid:            982

Truncated backtrace:
Thread no. 1 (10 frames)
 #2 log_assert_failed_realm at ../src/basic/log.c:806
 #3 dns_transaction_free at ../src/resolve/resolved-dns-transaction.c:94
 #4 dns_transaction_gc at ../src/resolve/resolved-dns-transaction.c:147
 #5 dns_transaction_complete at ../src/resolve/resolved-dns-transaction.c:366
 #6 on_transaction_stream_error at ../src/resolve/resolved-dns-transaction.c:473
 #7 on_stream_complete at ../src/resolve/resolved-dns-transaction.c:543
 #8 dns_stream_complete at ../src/resolve/resolved-dns-stream.c:62
 #9 source_dispatch at ../src/libsystemd/sd-event/sd-event.c:3111
 #10 sd_event_dispatch at ../src/libsystemd/sd-event/sd-event.c:3516
 #11 sd_event_run at ../src/libsystemd/sd-event/sd-event.c:3573