benchmark.cc: Simplify fcv() and dcv().
benchmark.cc: Simplify fcv() and dcv().

Part of the code already assumes that fcv() and dcv() update buffer.
Instead of calling them again, we can simply compare bufferown and
buffer to verify that Ryu and Grisu3 produce identical output.

ulfjack/ryu,ulfjack/ryu,ulfjack/ryu,ulfjack/ryu