pginstaller - pginst: Correctly check for NULL return value when

Log Message:
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Correctly check for NULL return value when checking the environment for SESSIONNAME to determine if a Terminal Services session is used.

This fixes the crash-before-install on Windows 2000 Professional and NT4 (!= TSE).

Modified Files:
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pginst/ca:
pginstca.c (r1.34 -> r1.35)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pginstaller/pginst/ca/pginstca.c.diff?r1=1.34&r2=1.35)