Fix errors reported by static code analyser

This fixes some errors reported by static code analyser:

• Possible memory leak in examples/samplesftp.c and in tests/unittests/torture_pki_dsa.c
• Adds null checks in tests:
  • torture_config.c
  • torture_options.c
  • torture_pki_dsa.c
  • torture_pki_ecdsa.c
  • torture_pki_ed25519.c
  • torture_pki_rsa.c
  • torture_threads_pki_rsa.c