One build 270 crash log aborts in talloc (talloc_abort_unknown_value)
inside libsmbclient name resolution while a media-source download opens
a file through SMBBridge, with FFmpeg reading an smb:// file on another
thread at the same time.
libsmbclient keeps its talloc stackframe behind a table of thread
functions, and none was installed (this build does not export
smbc_thread_posix()), so every thread shared one stackframe and the two
users freed each other's allocations. Install a pthread implementation
of smb_thread_functions at load time, before any libsmbclient use,
including FFmpeg's. The struct layout matches what talloc_stack and
smb_threads in the bundled library read (get_tls at +0x30, lock_mutex
at +0x10).