Correct a major read-past-buffer-end issue with the fixed_writer.
Correct a major read-past-buffer-end issue with the fixed_writer.

The length of the input was not being checked, so the fixed_writer would read up to its remaining length from input strings.

In the event that this didn't cause a crash, it would often corrupt the buffer, possibly just by filling it with excess garbage, causing excess writes to be ignored.

seanmiddleditch/formatxx