fix --stdout-errors handling

When using --stdout-errors, the messages written to stdout contained garbage.

Inside vfprintf(), each va_arg() consumes one entry from the va_args.
Trying to use the same va_args variable again results in undefined behavior
as subsequent va_arg() invocations continue to read from memory past the
actual variable space.

Instead, a copy has to be made with va_copy() and this be used for
outputting to stdout.