Check event loop creation return value. Fix #3951.

Normally we never check for OOM conditions inside Redis since the
allocator will always return a pointer or abort the program on OOM
conditons. However we cannot have control on epool_create(), that may
fail for kernel OOM (according to the manual page) even if all the
parameters are correct, so the function aeCreateEventLoop() may indeed
return NULL and this condition must be checked.

src/server.c

@@ -1782,6 +1782,12 @@ void initServer(void) {
     createSharedObjects();
     adjustOpenFilesLimit();
     server.el = aeCreateEventLoop(server.maxclients+CONFIG_FDSET_INCR);
+    if (server.el == NULL) {
+        serverLog(LL_WARNING,
+            "Failed creating the event loop. Error message: '%s'",
+            strerror(errno));
+        exit(1);
+    }
     server.db = zmalloc(sizeof(redisDb)*server.dbnum);
 
     /* Open the TCP listening socket for the user commands. */