pgsql: Use newly added InvalidCommandId instead of 0

Use newly added InvalidCommandId instead of 0

The symbol was added by 71901ab6d; the original code was introduced by
6868ed749.  Development of both overlapped which is why we apparently
failed to notice.

This is a (very slight) behavior change, so I'm not backpatching this to
9.4 for now, even though the symbol does exist there.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/6f822952eea112d3e19a0ddaad32e99d1943f29d

Modified Files
--------------
src/backend/access/heap/heapam.c |    6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)