audit callbacks from mboxlist_findall() and friends for handling of data->mbname

[elliefm]

The mboxlist_findall() and related APIs will call the callback under a number of different situations:

• a mailbox exactly matches the pattern(s)
• a mailbox partially matches the pattern(s) due to being a child of a matching mailbox
• a tombstone record matches the pattern(s)
• other cases?

Depending on exactly how the pattern was matched, the callback may be called with its struct findall_data *data argument in various states:

• data = NULL
• data->mbname = NULL
• data->mbentry = NULL

We keep getting issues where sections of code are naively not expecting partial matches, and then crashing because they depend on something being in the data struct which isn't.

So perhaps instead of fixing these piecemeal as they come up, we should actually audit all the callers of the mboxlist_findall() APIs to ensure their callback functions deal with partial matches sanely (in most cases, probably just by explicitly ignoring them, rather than crashing!).

[elliefm]

This was taken care of by 6ecbeda