BUG/MEDIUM: stats: properly initialize the scope before dumping stats

Issuing a "show sess all" prior to a "show stat" on the CLI results in no
proxy being dumped because the scope_len union member was not properly
reinitialized.

This fix must be backported into 1.5.
(cherry picked from commit 6bcb95da5b9cb143088102b460c7bcb37c1b3d81)
diff --git a/src/dumpstats.c b/src/dumpstats.c
index b616478..ca084ac 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -1109,6 +1109,8 @@
 		arg++;
 	}
 
+	appctx->ctx.stats.scope_str = 0;
+	appctx->ctx.stats.scope_len = 0;
 	appctx->ctx.stats.flags = 0;
 	if (strcmp(args[0], "show") == 0) {
 		if (strcmp(args[1], "stat") == 0) {