epoll: Check if fileref is valid

epoll_ctl can happpen while someone cloeses the fd concurrently.
Do nothing if fileref is empty.

Saw this happens in tomcat benchmark.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
enhance symbol detection script

Our symbol script has problem detecting symbols that has the form GLIBCXX.
In particular, because they will be a match in the grep part of the expression
but won't be replaced in the sed part of the expression, there will be garbage
in the output, leading the script to incorrectly guess which symbols are missing

Expanding the regular expression to also match XX symbols fix this.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>