util/log: Fix log messages over 1024 characters.

util/log: Fix log messages over 1024 characters.

The first attempt at the sprintf would have consumed part of va, so if
we're going to recurse on overflow to try again in a new allocation then
we have to do our work on a copy.

This was a common failure mode for MESA_GLSL=source, where it would just print:

  Mesa: info: GLSL source for fragment shader 1:
  Mesa: info: (null)

Fixes: 7a18a1712a0a ("util/log: improve logger_file newline handling")
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22618>