Events: fixed null pointer dereference with resolver and poll.

A POLLERR signalled by poll() without POLLIN/POLLOUT, as seen on
Linux, would generate both read and write events, but there's no
write event handler for resolver events.  A fix is to only call
event handler of an active event.

src/event/modules/ngx_poll_module.c

diff --git a/src/event/modules/ngx_poll_module.c b/src/event/modules/ngx_poll_module.c
index ed64bc5..4d45218 100644
--- a/src/event/modules/ngx_poll_module.c
+++ b/src/event/modules/ngx_poll_module.c
@@ -371,7 +371,7 @@
 
         found = 0;
 
-        if (revents & POLLIN) {
+        if ((revents & POLLIN) && c->read->active) {
             found = 1;
 
             ev = c->read;
@@ -388,7 +388,7 @@
             ngx_locked_post_event(ev, queue);
         }
 
-        if (revents & POLLOUT) {
+        if ((revents & POLLOUT) && c->write->active) {
             found = 1;
             ev = c->write;