[mod_sofia] Fix use-after-free in dispatch event thread. (#3031)

[mod_sofia] Fix use-after-free in dispatch event thread. (#3031)

`sofia_process_dispatch_event_in_thread` allocated `td` from a memory pool,
then `sofia_msg_thread_run_once` destroyed that same pool after processing
the event — leaving `td` dangling when the thread pool worker accessed it.

Allocate `td` with `switch_zmalloc` (`td->alloc = 1`) so the worker frees it
safely after the function returns. Remove the now-unused `pool` field from
`sofia_dispatch_event_t`.