options.c: Fix segv if poptGetContext returns NULL

[sahlberg]

If poptGetContext returns NULL, perhaps due to OOM, a NULL pointer is passed into poptReadDefaultConfig() which in turns SEGVs when trying to dereference it.

This was found using https://github.com/sahlberg/malloc-fail-tester.git

$ ./test_malloc_failure.sh rsync -Pav crash crosh

Need to test 4869
4891 allocations for rsync -Pav crash crosh
./test_malloc_failure.sh: line 9: 549305 Segmentation fault      (core dumped) ALLOC_FAIL=${IDX} LD_PRELOAD=./ld_alloc.so $PROGRAM $@ > /dev/null 2>&1
Crash at allocation #0
           PID: 549305 (rsync)
           UID: 1000 (sahlberg)
           GID: 1000 (sahlberg)
        Signal: 11 (SEGV)
     Timestamp: Thu 2025-01-30 13:29:58 AEST (721ms ago)
  Command Line: rsync -Pav crash crosh
    Executable: /usr/bin/rsync
 Control Group: /user.slice/user-1000.slice/[email protected]/app.slice/app-org.gnome.Terminal.slice/vte-spawn-90cea7ac-a067-4bff-9d0e-a8ef44417922.scope
          Unit: [email protected]
     User Unit: vte-spawn-90cea7ac-a067-4bff-9d0e-a8ef44417922.scope
         Slice: user-1000.slice
     Owner UID: 1000 (sahlberg)
       Boot ID: 51235d2adce14917828bb9d62b9bd051
    Machine ID: 94b9e1dd726e4cc9a0fb0dc93b9a801a
      Hostname: fedora
       Storage: /var/lib/systemd/coredump/core.rsync.1000.51235d2adce14917828bb9d62b9bd051.549305.1738207798000000.zst (present)
  Size on Disk: 103.8K
       Package: rsync/3.3.0-1.fc40
      build-id: fd1891af7f6287d01d804c8932b4847ed95206f5
       Message: Process 549305 (rsync) of user 1000 dumped core.

                Module libz.so.1 from rpm zlib-ng-2.1.7-2.fc40.x86_64
                Module libattr.so.1 from rpm attr-2.5.2-3.fc40.x86_64
                Module libcrypto.so.3 from rpm openssl-3.2.2-3.fc40.x86_64
                Module libxxhash.so.0 from rpm xxhash-0.8.3-1.fc40.x86_64
                Module libzstd.so.1 from rpm zstd-1.5.6-1.fc40.x86_64
                Module liblz4.so.1 from rpm lz4-1.9.4-6.fc40.x86_64
                Module libpopt.so.0 from rpm popt-1.19-6.fc40.x86_64
                Module libacl.so.1 from rpm acl-2.3.2-1.fc40.x86_64
                Module rsync from rpm rsync-3.3.0-1.fc40.x86_64
                Stack trace of thread 549305:
                #0  0x00007f5b097f2f67 poptReadDefaultConfig (libpopt.so.0 + 0xaf67)
                #1  0x000055bd526684d2 parse_arguments (rsync + 0x374d2)
                #2  0x000055bd52637f77 main (rsync + 0x6f77)
                #3  0x00007f5b09039088 __libc_start_call_main (libc.so.6 + 0x2a088)
                #4  0x00007f5b0903914b __libc_start_main@@GLIBC_2.34 (libc.so.6 + 0x2a14b)
                #5  0x000055bd5263b5b5 _start (rsync + 0xa5b5)
                ELF object binary architecture: AMD x86-64
NUM_CRASHES:1