tx.Check() crashes in read-only database

[rgeronimi]

When the database db is opened in read-only mode (options = bolt.Options{ReadOnly: true}),
and the transaction tx is created in read-only mode (tx, ... = db.Begin(false))

Then calls to tx.Check() will systematically crash the process (stack traces at bottom of this message).
It seems the freelist management code is not entirely aware of limitations of the read-only mode - more particularly nil instances.

---

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x10c21c2]

goroutine 18 [running]:
github.com/coreos/bbolt.(*freelist).free_count(...)
xxx/src/github.com/coreos/bbolt/freelist.go:52
github.com/coreos/bbolt.(*freelist).count(0x0, 0x0)
xxx/src/github.com/coreos/bbolt/freelist.go:47 +0x22
github.com/coreos/bbolt.(*Tx).check(0xc4200ca000, 0xc4200ce000)
xxx/src/github.com/coreos/bbolt/tx.go:396 +0xdb
created by github.com/coreos/bbolt.(*Tx).Check
xxx/src/github.com/coreos/bbolt/tx.go:389 +0x67

[lorneli]

When db is in read-only mode, db.freelist is not initialized in Open function. https://github.com/coreos/bbolt/blob/master/db.go#L254