Fix redirection of stderr in sys::Program::ExecuteAndWait. There was logic

error that caused it to redirect stderr to stdout too often.

This fix is applied identically to the win32 code as well, but that is
untested.
--Thi line, and those below, will be ignored--

M    System/Unix/Program.inc
M    System/Win32/Program.inc

llvm-svn: 52233