BUG/MEDIUM: peers: fix use after free in peer_session_create()

In case of resource allocation error, peer_session_create() frees
everything allocated and returns a pointer to the stream/session that
was put back into the free pool. This stream/session is then assigned
to ps->{stream,session} with no error control. This means that it is
perfectly possible to have a new stream or session being both used for
a regular communication and for a peer at the same time.

In fact it is the only way (for now) to explain a CLOSE_WAIT on peers
connections that was caught in this dump with the stream interface in
SI_ST_CON state while the error field proves the state ought to have
been SI_ST_DIS, very likely indicating two concurrent accesses on the
same area :

  0x7dbd50: [31/Oct/2016:17:53:41.267510] id=0 proto=tcpv4
    flags=0x23006, conn_retries=0, srv_conn=(nil), pend_pos=(nil)
    frontend=myhost2 (id=4294967295 mode=tcp), listener=? (id=0)
    backend=<NONE> (id=-1 mode=-) addr=127.0.0.1:41432
    server=<NONE> (id=-1) addr=127.0.0.1:8521
    task=0x7dbcd8 (state=0x08 nice=0 calls=2 exp=<NEVER> age=1m5s)
    si[0]=0x7dbf48 (state=CLO flags=0x4040 endp0=APPCTX:0x7d99c8 exp=<NEVER>, et=0x000)
    si[1]=0x7dbf68 (state=CON flags=0x50 endp1=CONN:0x7dc0b8 exp=<NEVER>, et=0x020)
    app0=0x7d99c8 st0=11 st1=0 st2=0 applet=<PEER>
    co1=0x7dc0b8 ctrl=tcpv4 xprt=RAW data=STRM target=PROXY:0x7fe62028a010
        flags=0x0020b310 fd=7 fd.state=22 fd.cache=0 updt=0
    req=0x7dbd60 (f=0x80a020 an=0x0 pipe=0 tofwd=0 total=0)
        an_exp=<NEVER> rex=<NEVER> wex=<NEVER>
        buf=0x78a3c0 data=0x78a3d4 o=0 p=0 req.next=0 i=0 size=0
    res=0x7dbda0 (f=0x80402020 an=0x0 pipe=0 tofwd=0 total=0)
        an_exp=<NEVER> rex=<NEVER> wex=<NEVER>
        buf=0x78a3c0 data=0x78a3d4 o=0 p=0 rsp.next=0 i=0 size=0

Special thanks to Arnaud Gavara who provided lots of valuable input and
ran some validation testing on this patch.

This fix must be backported to 1.6 and 1.5. Note that in 1.5 the
session is not assigned from within the function so some extra checks
may be needed in the callers.

src/peers.c

@@ -1845,7 +1845,7 @@ static struct stream *peer_session_create(struct peers *peers, struct peer *peer
  out_free_appctx:
 	appctx_free(appctx);
  out_close:
-	return s;
+	return NULL;
 }
 
 /*