BUG/MINOR: acme: possible integer underflow in acme_txt_record()

BUG/MINOR: acme: possible integer underflow in acme_txt_record()

a2base64url() can return a negative value is olen is too short to
accept ilen. This is not supposed to happen since the sha256 should
always fit in a buffer. But this is confusing since a2base64()
returns a signed integer which is pt in output->data which is unsigned.

Fix the issue by setting ret to 0 instead of -1 upon error. And returns
a unsigned integer instead of a signed one.
This patch also checks the return value from the caller in order
to emit an error instead of setting trash.data which is already done
from the function.