coredump/stacktrace.c: avoid crash on binaries without NHDR

Observed as a crash on binaries built with gcc-master:

```
 3  0x00005573b8841d6a in parse_package_metadata (name=name@entry=0x5573b901a620 "/a", id_json=0x0,
    elf=0x5573b9028730, c=c@entry=0x7fffc688f790) at ../systemd-stable-249.2/src/coredump/stacktrace.c:157
 4  0x00005573b884209e in module_callback (mod=0x7fffc688f6c0, userdata=<optimized out>,
    name=0x5573b901a620 "/a", start=<optimized out>, arg=0x7fffc688f790)
    at ../systemd-stable-249.2/src/coredump/stacktrace.c:306
 5  0x00007f56d60dcbd5 in dwfl_getmodules (dwfl=0x5573b901fda0,
    callback=callback@entry=0x5573b8841eb0 <module_callback>, arg=arg@entry=0x7fffc688f790, offset=offset@entry=0)
    at ../../elfutils-0.185/libdwfl/dwfl_getmodules.c:86
 6  0x00005573b884231b in parse_core (ret_package_metadata=0x7fffc688f848, ret=0x7fffc688f850,
    executable=0x7fffc688f790 "\200\332\001\271sU", fd=21) at ../systemd-stable-249.2/src/coredump/stacktrace.c:366
 7  coredump_parse_core (fd=fd@entry=6, executable=0x7fffc688f790 "\200\332\001\271sU", ret=ret@entry=0x7fffc688f850,
    ret_package_metadata=ret_package_metadata@entry=0x7fffc688f848)
    at ../systemd-stable-249.2/src/coredump/stacktrace.c:406
 8  0x00005573b883f897 in submit_coredump (context=context@entry=0x7fffc688fa10, iovw=iovw@entry=0x7fffc688f990,
    input_fd=input_fd@entry=5) at ../systemd-stable-249.2/src/coredump/coredump.c:827
 9  0x00005573b883d339 in process_socket (fd=3) at ../systemd-stable-249.2/src/coredump/coredump.c:1041
 10 run (argv=<optimized out>, argc=-964101648) at ../systemd-stable-249.2/src/coredump/coredump.c:1416
 11 main (argc=-964101648, argv=<optimized out>) at ../systemd-stable-249.2/src/coredump/coredump.c:1422
```

Happens only on enabled elfutils symbolizer.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>