Fix NULL dereferencing

[trivialkettle]

Solves #19

[arobenko]

The suggested fix is incorrect, the loop above has already found the right session object. However, when the posted function is executed, the sessions list is already modified and the iterator is pointing to an invalid memory. It's just by coincidence the suggested condition is evaluated to false. The proper approach would be erasing the found session object right away without posting. Due to the fact this that erasing happens to the different session object than the one invoking the callback, it should be safe to do it from within a callback.