A couple of places were missing NULL return value checks for
mdb_read_table()'s return value. Add these.
This fixes a NULL pointer deref while running ./test_script.sh
on the test mdb file from oss-fuzz/35972 .
Note this does NOT fix the original problem reported in oss-fuzz/35972
which reports a "Dynamic-stack-buffer-overflow WRITE 16" issue,
which I've been unable to reproduce.