Bug 661792 - Valid managed entry config rejected
    
    The attribute mapping parsing code in the managed entries plug-in
    can access the wrong memory when trying to parse the post-macro
    portion of a mapping value when no post string exists.  When the
    macro is at the end of the mapping value, we were setting post_str
    to an empty constant string.  The code that later parses post_str
    expects post_str to be contained within the actual mapping value
    string.  This is not the case when we have set post_str to point
    to a constant emptry string, so we end up parsing through memory
    that we shouldn't.
    
    The fix is to set post_str to NULL when there is no post string.
    We can then skip post string parsing when post_str is not set.