BUG/MINOR: mux-spop: Make the demux stream ID a signed integer

The demux stream ID of a SPOP connection, used when received frames are
parsed, must be a signed integer because it is set to -1 when the SPOP
connection is initialized. It will be important for the next fix.

This patch must be backported to 3.1.

Author: capflam

src/mux_spop.c

@@ -31,7 +31,7 @@ struct spop_conn {
 	uint32_t max_id;                     /* highest ID known on this connection, <0 before HELLO handshake */
 	uint32_t flags;                      /* Connection flags: SPOP_CF_* */
 
-	uint32_t dsi;                        /* dmux stream ID (<0 = idle ) */
+	int32_t dsi;                         /* dmux stream ID (<0 = idle ) */
 	uint32_t dfi;                        /* dmux frame ID (if dsi >= 0) */
 	uint32_t dfl;                        /* demux frame length (if dsi >= 0) */
 	uint32_t dff;                        /* demux frame flags */