pgrpm - pgrpm: - return an empty string in case rpmds return null ptr

Log Message:
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- return an empty string in case rpmds return null ptr

Modified Files:
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pgrpm:
pgheader.c (r1.20 -> r1.21)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgrpm/pgrpm/pgheader.c.diff?r1=1.20&r2=1.21)