Python - Fix segfault when printing empty 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