fix: nil pointer dereference in ScanTLS when no cert has DNSNames

[larinkonstantin]

Fixes #39

PROBLEM
ScanTLS panics with a nil pointer dereference when none of the peer certificates contain DNSNames (e.g. certificates that only use Subject.CommonName).

panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x2 addr=0x90 pc=0x102cd96f0]

  goroutine 11 [running]:
  main.ScanTLS(...)
          scanner.go:76

ROOT CAUSE
leaf was declared as var leaf *x509.Certificate and only assigned inside the loop if a certificate had DNSNames. If no such certificate existed, leaf remained nil. It was then unconditionally dereferenced in the slog call at the end of the function, even for non-feasible hosts.

FIX
Initialize leaf to the first certificate before the loop and override it with the first certificate that has DNSNames if one is found.

[yuhan6665]

Thanks for your fix!