Unhandled exception from tlsext_servername_callback generates SIGSEGV.

If the function specified by set_tlsext_servername_callback generates an exception, this code will generate a SEGFAULT on context.c:270.

This changes the handling of the return value for this function to remain consistent with the other callbacks in this file: if an exception is returned, silently absorb the error and do not call Py_DECREF(ret).