pgsql: Tweak toast-related logic in heapam.c so that the toaster is only

Log Message:
-----------
Tweak toast-related logic in heapam.c so that the toaster is only invoked
when relkind = RELKIND_RELATION.  This syncs these tests with the Asserts
in tuptoaster.c, and ensures that we won't ever try to, for example,
compress a sequence's tuple.  Problem found by Greg Stark while stress-testing
with much-smaller-than-normal page sizes.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        heapam.c (r1.242 -> r1.243)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.242&r2=1.243)