bug_fl(): correctly initialize trace2 va_list

bug_fl(): correctly initialize trace2 va_list

The code added 0cc05b044f (usage.c: add a non-fatal bug() function to go
with BUG(), 2022-06-02) sets up two va_list variables: one to output to
stderr, and one to trace2. But the order of initialization is wrong:

  va_list ap, cp;
  va_copy(cp, ap);
  va_start(ap, fmt);

We copy the contents of "ap" into "cp" before it is initialized, meaning
it is full of garbage. The two should be swapped.

However, there's another bug, noticed by Johannes Schindelin: we forget
to call va_end() for the copy. So instead of just fixing the copy's
initialization, let's do two separate start/end pairs. This is allowed
by the standard, and we don't need to use copy here since we have access
to the original varargs. Matching the pairs with the calls makes it more
obvious that everything is being done correctly.

Note that we do call bug_fl() in the tests, but it didn't trigger this
problem because our format string doesn't have any placeholders. So even
though we were passing a garbage va_list through the stack, nobody ever
needed to look at it. We can easily adjust one of the trace2 tests to
trigger this, both for bug() and for BUG(). The latter isn't broken, but
it's nice to exercise both a bit more. Without the fix in this patch
(but with the test change), the bug() case causes a segfault.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>