Hi,
Thanks for finding those issues.  I attached a new patch.

a) Good find, I added the free() for gdbm.  ndbm doesn't need free().
b) Added the error check.  I don't know if a test can be made for this.  If there was a common way to patch C libraries in CPython, I would give that a try.
Also, do you know if we should check for the gdbm error before the dbm error, or does it not matter?
c) Yes, it looks like NULL is used here instead of 0.  Changed 0s to NULLS.

Let me know if you see anything else.