pldmtool: platform: check for circular references

Avoid endless loops with:

pldmtool platform getpdr -a

by detecting circular references between records in the event of a
buggy/broken PLDM terminus implementation.

Change-Id: I2299c4d709fa0b8861956e55ada38da8fffb2d6e
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>