pextlib1.0: init err buffer, use err code if empty
pextlib1.0: init err buffer, use err code if empty

curl < 7.60.0 does not initialize the error buffer, and cases have been
reported where a curl download failed without the error buffer being
set. This was the case on older systems that did not yet have curl
7.60.0, and caused random garbage to be printed.

0-initialize the buffer and check whether the buffer has a useful
message before returning it. Generate a substitute message using
curl_easy_strerror() from the returned error code if the error buffer is
not set.

Closes: https://trac.macports.org/ticket/60581

macports/macports-base,neverpanic/macports-base,danchr/macports-base,neverpanic/macports-base,danchr/macports-base,macports/macports-base