BUG/MINOR: stats: fix show stats field ctx for servers

In ("MINOR: stats: introduce stats field ctx"), we forgot
to apply the patch to servers.

This prevents "BUG/MINOR: stats: fix show stat json buffer limitation"
from working with servers dump.

We're adding the missing part related to servers dump.

This commit should be backported with the aforementioned commits.

src/stats.c

@@ -3311,7 +3311,10 @@ int stats_dump_proxy_to_buffer(struct stconn *sc, struct htx *htx,
 			if (stats_dump_sv_stats(sc, px, sv)) {
 				if (!stats_putchk(rep, htx))
 					goto full;
+				if (ctx->field)
+					goto more;
 			}
+			current_field = 0;
 		} /* for sv */
 
 		ctx->px_st = STAT_PX_ST_BE;