[BUG] stats: admin commands must check the proxy state

As reported by Bryan Talbot, enabling and disabling a server in a disabled
proxy causes a segfault.

Changing the weight can also cause a similar segfault.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index 980b284..081e4cd 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -639,6 +639,12 @@
 				return 1;
 			}
 
+			if (px->state == PR_STSTOPPED) {
+				s->data_ctx.cli.msg = "Proxy is disabled.\n";
+				si->st0 = STAT_CLI_PRINT;
+				return 1;
+			}
+
 			/* if the weight is terminated with '%', it is set relative to
 			 * the initial weight, otherwise it is absolute.
 			 */
@@ -749,6 +755,12 @@
 				return 1;
 			}
 
+			if (px->state == PR_STSTOPPED) {
+				s->data_ctx.cli.msg = "Proxy is disabled.\n";
+				si->st0 = STAT_CLI_PRINT;
+				return 1;
+			}
+
 			if (sv->state & SRV_MAINTAIN) {
 				/* The server is really in maintenance, we can change the server state */
 				if (sv->tracked) {
@@ -804,6 +816,12 @@
 				return 1;
 			}
 
+			if (px->state == PR_STSTOPPED) {
+				s->data_ctx.cli.msg = "Proxy is disabled.\n";
+				si->st0 = STAT_CLI_PRINT;
+				return 1;
+			}
+
 			if (! (sv->state & SRV_MAINTAIN)) {
 				/* Not already in maintenance, we can change the server state */
 				sv->state |= SRV_MAINTAIN;