Add asserts in trace

[Foxpunk]

Problem

Several switch statements over enum-like fields, and return values of
time(), fseek() and snprintf(), were not checked, which could lead to
silent incorrect behavior.

Fix

Added explicit default: fb_assert(false) branches to switch
statements to catch unexpected values early.
Saved the return values of time(), fseek() and snprintf() into
[[maybe_unused]] variables and added fb_assert checks on them.

[craftmaster1231]

Approved.