Volatile read of the VCL temperature

There is currently no locking around temperature changes despite the
fact that it may be read outside of the CLI thread to make decisions.

In the case of a panic, instead of locking we simply make sure not to
perform an incomplete read of the pointer.

bin/varnishd/cache/cache_vcl.c

@@ -127,7 +127,7 @@ VCL_Panic(struct vsb *vsb, const struct vcl *vcl)
 	VSB_printf(vsb, "busy = %u\n", vcl->busy);
 	VSB_printf(vsb, "discard = %u,\n", vcl->discard);
 	VSB_printf(vsb, "state = %s,\n", vcl->state);
-	VSB_printf(vsb, "temp = %s,\n", vcl->temp);
+	VSB_printf(vsb, "temp = %s,\n", (const volatile char *)vcl->temp);
 	VSB_printf(vsb, "conf = {\n");
 	VSB_indent(vsb, 2);
 	if (vcl->conf == NULL) {