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Description of problem:

rsyslogd segfault when set statement is not valid.

# cat /etc/rsyslog.d/test.conf
set $testvar;
# rsyslogd -n -d
Segmentation fault
# rpm -q rsyslog
rsyslog-8.24.0-12.el7.x86_64

Same result also for statement
set $testvar=;

gdb says:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff3cd0700 (LWP 11806)]
0x00007ffff639f4b7 in readjournal () at imjournal.c:326
326		if (sys_iden == NULL && !cs.dfltTag[0]) {

This is a regression against the upstream version.
I have confirmed that the issue is caused by our downstream patch 
  rsyslog-8.24.0-rhbz1188503-imjournal-default-tag.patch

Created attachment 1304172 [details]
proposed patch

Some guy was dereferencing pointer which was NULL in that specific case.
I fixed that condition but some addition tests are required because I am not sure if it doesn't change functionality in other use cases.

I think that previous version check only if default tag was empty string and doesn't check what this pointer was valid. Now the condition checks whether pointer is valid but not occurrence of the empty string. 

I think that condition should cover both cases.

Created attachment 1304713 [details]
proposed patch v2

Added string check as well.

Created attachment 1319072 [details]
final patch

*** Bug 1509987 has been marked as a duplicate of this bug. ***

*** Bug 1553599 has been marked as a duplicate of this bug. ***

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:0856