perlbug: quit main loop on empty answer / eof in test mode

This makes it possible to drive perlbug noninteractively
without having it go to an infinite loop on end of file.

The change has no effect in non-test mode, where the default answer is
the empty string.

This is groundworks for perlbug unit tests.

Author: ntyni

utils/perlbug.PL

@@ -828,7 +828,8 @@ a few options. You can:
 EOF
       retry:
         print $menu;
-	    my $action =  _prompt('', "Action (Send/Display/Edit/Subject/Save to File)");;
+	    my $action =  _prompt('', "Action (Send/Display/Edit/Subject/Save to File)",
+	        $opt{t} ? 'q' : '');
         print "\n";
 	    if ($action =~ /^(f|sa)/i) { # <F>ile/<Sa>ve
             if ( SaveMessage() ) { exit }
@@ -1014,6 +1015,7 @@ sub _prompt {
     }
     print $prompt. ($default ? " [$default]" :''). ": ";
 	my $result = scalar(<>);
+    return $default if !defined $result; # got eof
     chomp($result);
 	$result =~ s/^\s*(.*?)\s*$/$1/s;
     if ($default && $result eq '') {