About the bug (1): it also occurs in DBEnv_dealloc() but 
DBEnv_dealloc() is directly called from newDBEnvObject() with 
Py_DECREF(self);. The two bugs can be reproduces with dummy DBenv() 
arguments, eg. "DBEnv(92)".

Backtrace using gdb:
-----
$ gdb ./python
...
>>> import _bsddb; _bsddb.DBenv(92)
...
Program received signal SIGSEGV, Segmentation fault.

0xb7cc8f5e in DBEnv_close_internal (self=0x83385f0, flags=0) 
at ../Modules/_bsddb.c:3989
3989            err = self->db_env->close(self->db_env, flags);
(gdb) print self->db_env
$1 = (DB_ENV *) 0xfffffffe
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There db_env value is not set.