Fix tests broken by #206e1af.

This fixes issue #26 All tests and benchmarks pass (MacBook Pro Retina, 2.3GHz i7, SSD):

```
~/go/src/github.com/szferi/gomdb [master ⚡] % go test -v
# github.com/szferi/gomdb
./mdb.c:8605:46: warning: data argument not used by format string [-Wformat-extra-args]
/usr/include/secure/_stdio.h:47:56: note: expanded from macro 'sprintf'
=== RUN TestEnvOpen
--- PASS: TestEnvOpen (0.00 seconds)
=== RUN TestEnvCopy
--- PASS: TestEnvCopy (0.00 seconds)
	env_test.go:67: Env path: /tmp/mdb_test749479226
=== RUN TestErrno
--- PASS: TestErrno (0.00 seconds)
=== RUN TestTest1
--- PASS: TestTest1 (0.00 seconds)
	mdb_test.go:67: &{PSize:4096 Depth:1 BranchPages:0 LeafPages:1 OverflowPages:0 Entries:10}
	mdb_test.go:91: Val: Val-0
	mdb_test.go:92: Key: Key-0
	mdb_test.go:91: Val: Val-1
	mdb_test.go:92: Key: Key-1
	mdb_test.go:91: Val: Val-2
	mdb_test.go:92: Key: Key-2
	mdb_test.go:91: Val: Val-3
	mdb_test.go:92: Key: Key-3
	mdb_test.go:91: Val: Val-4
	mdb_test.go:92: Key: Key-4
	mdb_test.go:91: Val: Val-5
	mdb_test.go:92: Key: Key-5
	mdb_test.go:91: Val: Val-6
	mdb_test.go:92: Key: Key-6
	mdb_test.go:91: Val: Val-7
	mdb_test.go:92: Key: Key-7
	mdb_test.go:91: Val: Val-8
	mdb_test.go:92: Key: Key-8
	mdb_test.go:91: Val: Val-9
	mdb_test.go:92: Key: Key-9
=== RUN TestVal
--- PASS: TestVal (0.00 seconds)
=== RUN: Example
--- PASS: Example (870.656us)
PASS
ok  	github.com/szferi/gomdb	0.022s
~/go/src/github.com/szferi/gomdb [master ⚡] % go test -bench . 
# github.com/szferi/gomdb
./mdb.c:8605:46: warning: data argument not used by format string [-Wformat-extra-args]
/usr/include/secure/_stdio.h:47:56: note: expanded from macro 'sprintf'
PASS
BenchmarkTxnPut	  500000	      2387 ns/op
--- BENCH: BenchmarkTxnPut
	bench_test.go:301: initializing random source data
BenchmarkTxnGetRDONLY	 1000000	      1592 ns/op
BenchmarkTxnGetValRDONLY	 1000000	      1151 ns/op
BenchmarkCursorScanRDONLY	      10	 130547727 ns/op
BenchmarkCursorScanValRDONLY	      50	  34778472 ns/op
ok  	github.com/szferi/gomdb	62.268s
```