Avoid segfault: Don't read and write into the same buffer.

Avoid segfault: Don't read and write into the same buffer.

Using CTX->rrd_error as a source *and* destination buffer at the same time
provokes undefined behavior.  In real life you get funny error messages
and/or segfaults within vs(n)printf().

Simple solution: Use a temporary copy of the error message while writing
into it.