Fix invalid free in trace_close_memstream

Fix invalid free in trace_close_memstream

In maybe_switch_tcbs we exchange the pointers to the memstream's buffers
between 2 tcb, however the libc doesn't know and keeps updating the
tcb->memfptr as if the exchange didn't happen.  This leads to
unsynchronized tcb->memfptr and tcb->outf and invalid frees.
Adding a new indirection fixes the problem.

* stage_output.c (struct staged_output_data): New struct.
(strace_open_memstream, strace_close_memstream): Use it.
* defs.h (struct tcb): Replace real_outf, memfptr, and memfloc
with a pointer to struct staged_output_data.
* strace.c (maybe_switch_tcbs): Use it.
* syscall.c (print_syscall_resume): Ditto.

Signed-off-by: Pierre Marsais <pierre.marsais@lse.epita.fr>