Problem: Program crashes if memory allocation in socket_poller_t::rebuild fails.

Solution: Report memory allocation failure as ENOMEM so applications can handle it gracefully.

Fixes #3427.

src/socket_poller.hpp

@@ -101,7 +101,7 @@ class socket_poller_t
                         uint64_t &now_,
                         uint64_t &end_,
                         bool &first_pass_);
-    void rebuild ();
+    int rebuild ();
 
     //  Used to check whether the object is a socket_poller.
     uint32_t _tag;