Hydra/18: fixed uninitialized value error when logfile is null

Dependencies

• [2] BOFOHCPK removed debug print, added last 50 lines in failure emails
• [3] QUTWJR7P * Include more info in notification emails.
• [4] KA45EBF5 * Send email if a build fails.
• [*] 7YBYT2LQ

Change contents

• replacement in src/script/hydra_build.pl at line 151
  [2.71]→[2.71:132](∅→∅)
  − my $logtext = `tail -$loglines $logfile` if -e $logfile;
  [2.71]

  [3.807]
  + my $logtext = defined $logfile && -e $logfile ? `tail -$loglines $logfile` : "No logfile available.\n";