perl plugin: Fix potential double-free.

perl plugin: Fix potential double-free.

The av2notification_meta() function updated it's parameter by doing

    (*m) = malloc();

Afterwards, if an error occurred, it would call `free(*m);` and (potentially)
return, leaving an invalid pointer stored in `m`. The caller would then try to
free the returned pointer because it was non-NULL.

This patch fixes this by doing several code cleanups:

*   The meta argument is not updated unnecessarily. Instead, a local variable
    is allocated and used within the loop and only on success is the return
    pointer updated.
*   Introduce additional local variables instead of re-using the `tmp` variable
    multiple times.
*   Name the variable pointing to the end of the linked list appropriately
    `tail` and remove one level of indirection. Reading code that is using
    pointers to pointers is unnecessarily hard.