base64: fix Curl_base64_encode and Curl_base64_decode interfaces

Previous interfaces for these libcurl internal functions did not allow to tell
apart a legitimate zero size result from an error condition. These functions
now return a CURLcode indicating function success or otherwise specific error.
Output size is returned using a pointer argument.

All usage of these two functions, and others closely related, has been adapted
to the new interfaces. Relative error and OOM handling adapted or added where
missing. Unit test 1302 also adapted.