This reverts commit 06a9efee.

This avoids redundant "if (x == NULL)" style checks and puts the onus on
the caller to use valid parameters.

In the case of errors depending on network input (i.e. not programming
logic errors), we should report errors but not abort the application.
Assertions should be used to validate and document programmer
assumptions. Moreover, if assertions are deactivated at build-time, we
don't want our application to crash.

We should only store and clean up stun sockets if
a) they don't already exist for the server in question
b) they are valid (created and started successfully)