Probable Incorrect Error Handling in collective_create()

Nobody has claimed this yet.

### System information

Type | Version/Name
--- | ---
Operating System | All of them, I suspect
OS Version |
Architecture |
UnifyFS Version |

### Describe the problem you're observing
In collective_create() (around about line 303) we attempt to acquire a child request handle. If we can't get a handle for some reason, we do **NOT** clean up and exit (as we do with most other error conditions). Instead, we log an error and set the pointer to NULL, but otherwise continue processing. If no other errors occur, then the function will return a pointer to a coll_request struct, but that struct will contain an unexpected NULL pointer.

### Describe how to reproduce the problem
The bug is in the error handling for this function, so you'd have to trigger the specific error. I'm not sure how you'd do that (and I rather suspect it's never happened on a production system).

### Include any warning or errors or releveant debugging data
This was discovered during a code inspection as part of the work on #758. For reference, Michael Brim's comment about it is reproduced below:

>Regarding the TODO, the reason we don't return immediately in that error case is because that would leak the allocated coll_request structure and its member array allocations. However, it looks like we're missing the logic to notice that we failed to get one of the child handles and need to cleanup the structure before returning NULL. Unfortunately, it appears it's not as easy as just calling collective_cleanup(), since that would prematurely free the original group rpc request handle, which we use in the callers to collective_create() to send back an error. This is complicated enough that we probably should open an issue and address in a separate PR.

Start by inspecting collective_create() around line 303 and the related collective_cleanup() behavior. Trace how callers use the original group RPC request handle when child-handle acquisition fails. Done means the failure cleans up the coll_request allocations, returns NULL, and preserves the handle needed by callers to report the error.

Written by the indexing model from the issue text.