Stab at fixing memcpy bug in WriteMemoryCallback

Do we want memcpy instead of mem->memory = tmp?

Setting CURL_WRITE_BUF to a small value exposes this codepath and a segfault.

Details; (Copying from a fix in sasl-browserid[1], not tested under mod_browserid, yet)
I assume we shouldn't call free and can just use apr_palloc twice.
[1] https://github.com/ozten/sasl-browserid/blob/master/plugins/verifier.c#L33