BUG/MINOR: spoa-example: unexpected behavior for more than 127 args

Buf is unsigned, so nbargs will be negative for more then 127 args.

Note that I cant test this bug because I cant put sufficient args
on the configuration line. It is just detected reading code.

[wt: this can be backported to 1.8 & 1.7]

contrib/spoa_example/spoa.c

@@ -1318,7 +1318,7 @@ process_frame_cb(evutil_socket_t fd, short events, void *arg)
 
 		DEBUG(frame->worker, "Process SPOE Message '%.*s'", (int)sz, str);
 
-		nbargs = *p++;                     /* Get the number of arguments */
+		nbargs = (unsigned char)*p++;      /* Get the number of arguments */
 		frame->offset = (p - frame->buf);  /* Save index to handle errors and skip args */
 		if (!memcmp(str, "check-client-ip", sz)) {
 			union spoe_data data;