python - be: If the call's iterator hits the end, it will return NULL

Log Message:
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If the call's iterator hits the end, it will return NULL without exception, so
handle this case simply by returning None. (Probably more context dependent
logic should be used to identify what should actually happen here. However, for now
this prevents a common invalid ERROR from being thrown.)

Modified Files:
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be/src/call:
pl.c (r1.9 -> r1.10)
(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/python/be/src/call/pl.c.diff?r1=1.9&r2=1.10)