micro optimization

Some micro optimizations to IPC::Run. With these two commits two test script below runs somewhat faster, about 10% on my system:

    srezic-micro-optimization: env IPCRUNDEBUG=none perl -Mblib /tmp/ipc-run-1000.pl 
    2.83188390731812
    master:                    env IPCRUNDEBUG=none perl -Mblib /tmp/ipc-run-1000.pl 
    3.12365984916687
    
    srezic-micro-optimization: perl -Mblib /tmp/ipc-run-1000.pl                  
    3.19277095794678
    master:                    perl -Mblib /tmp/ipc-run-1000.pl 
    3.4070770740509


     #!/usr/bin/perl
     
     use strict;
     use IPC::Run qw(run);
     use Time::HiRes qw(time);
     
     my $n = 1000;
     
     {
         my $t0 = time;
         run ['echo', 'foo'], '|', ['true']
     	for 1..$n;
         warn time-$t0, "\n";
     }
     __END__