Fixes Bug 1072540 - Look Ma! No pipe dump!

the TransformRule version of the stackwalker invocation without producing or needing a pipe dump!  
it would have been true if the ```self._invoke_minidump_stackwalk``` had declined to process a dump because the dump_name had the wrong prefix.  However, since your earlier objection, I moved the detection code to the beginning of the loop rather that within the invocation of stackwalker.  This line is now pretty much useless, I'll remove it.