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.

[hvlad]

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.

You should better look at the code and get understanding what it does.

Saved the return values of time(), fseek() and snprintf() into
[[maybe_unused]] variables and added fb_assert checks on them.

How this visual spam prevents "silent incorrect behavior" ? Sorry, can't resist.

[Foxpunk]

@hvlad I can drop the fb_assert(false) default branches from the switch
statements. I just thought it might help during testing, but I don't
have a strong enough argument for keeping them.

Can I keep the fb_assert checks on the time()/fseek()/snprintf() return
values, since those actually verify something concrete?

I'll also clean up the PR description to be more to the point.