Small python segfault fix when printing empty units structs

[Michele Baldessari <michele@xxxxxxxxxx>]

Hi all,

on https://github.com/mbaldessari/pcp/tree/fix-python-pmunitsstr 
you can find a small fix for a segfault I stumbled upon while using
the python bindings.

Do let me know if this way of sending patches is ok ;)

regards,
Michele

commit a01a5c131d047b98b5aa0bd9917614ce69f7d764
Author: Michele Baldessari <michele@xxxxxxxxxx>
Date:   Sat May 24 11:00:55 2014 +0100

    Python - Fix segfault when printing empy units
    
    The following example:
    """
    import pcp.pmapi
    from pcp.pmapi import c_api
    
    fname = '20140506.0'
    context = pcp.pmapi.pmContext(c_api.PM_CONTEXT_ARCHIVE, fname)
    pmids = context.pmLookupName(['proc.nprocs'])
    descs = context.pmLookupDescs(pmids[0])
    print 'Segfault: %s' % descs[0].contents.units
    """
    
    will segfault:
    pmUnitsStr_r (pu=0x0, buf=buf@entry=0x3d2e66f9c0 <ubuf> "", 
buflen=buflen@entry=60) at units.c:177
    177         if (pu->dimSpace) {
    
    Make sure we wrap pmUnitsStr in order to catch these and correctly
    return an empty string


-- 
Michele Baldessari            <michele@xxxxxxxxxx>
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