dprintf_formatf: fix (false?) Coverity warning

CID 1024412: Memory - illegal accesses (OVERRUN). Claimed to happen when
we run over 'workend' but the condition says <= workend and for all I
can see it should be safe. Compensating for the warning by adding a byte
margin in the buffer.

Also, removed the extra brace level indentation in the code and made it
so that 'workend' is only assigned once within the function.