coredump: do not crash if we failed to acquire exe path

The COREDUMP_EXE attribute is "optional", i.e. we continue to process the
crash even if we didn't acquire it. The coredump generation code assumed
that it is always available:

 #5 endswith at ../src/fundamental/string-util-fundamental.c:41
 [ endswith() is called with NULL here, and an assertion fails. ]
 #6 submit_coredump at ../src/coredump/coredump.c:823
 #7 process_socket at ../src/coredump/coredump.c:1038
 #8 run at ../src/coredump/coredump.c:1413

We use the exe path for loop detection, and also (ultimately) pass it to
dwfl_core_file_report(). The latter seems to be fine will NULL, so let's just
change our code to look at COMM, which should be more reliable anyway.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=2036517.

(cherry picked from commit c790632c)