Fixes Bug 1072540 - Look Ma! No pipe dump!

the TransformRule version of the stackwalker invocation without producing or needing a pipe dump!  
patch coming that moves this check closer to the beginning of the loop that supplies dumps to the stackwalker.  that patch includes this comment: 

``` python
                # this rule is only interested in dumps targeted for the
                # minidump stackwalker external program.  As of the writing
                # of this code, there is one other dump type.  The only way
                # to differentiate these dump types is by the name of the
                # dump.  All minidumps targeted for the stackwalker will have 
                # a name with a prefix specified in configuration:
                if not dump_name.startswith(self.config.dump_field):
                    continue
```