Add if statement, even if redundant

This short PR explores the possibility of adding a slightly redundant `if` statement...

The if statement is probably redundant since `rb_gc_mark` tests for special non-allocated "objects" such as `nil`, `false`, embedded numbers etc').

However: (1) since a NULL dereferencing issue was reported; and (2) GC tests this condition only after pushing the stack (so it's faster if we test for it); we might as well add this slightly redundant `if` statement and see if it helps.

Author: boazsegev

ext/nio4r/monitor.c

@@ -66,7 +66,8 @@ static VALUE NIO_Monitor_allocate(VALUE klass)
 
 static void NIO_Monitor_mark(struct NIO_Monitor *monitor)
 {
-    return rb_gc_mark(monitor->self);
+    if(monitor && monitor->self)
+        rb_gc_mark(monitor->self);
 }
 
 static void NIO_Monitor_free(struct NIO_Monitor *monitor)