segmentation fault using currtid and partitioned tables

Hi,

Another one caught by sqlsmith, on the regression database run this
query (using any non-partitioned table works fine):

"""
select currtid('pagg_tab'::regclass::oid, '(0,156)'::tid) >= '(1,158)'::tid;
"""

This works on 11 (well it gives an error because the file doesn't
exists) but crash the server on 12+

attached the stack trace from master

--
Jaime Casanova www.2ndQuadrant.com
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