Fix memory leaks in CTLOG_new_from_base64

Move the call to ct_base64_decode(), which allocates, until after
the check for NULL output parameter.

Also place a cap on the number of padding characters used to decrement
the output length -- any more than two '='s is not permitted in a
well-formed base64 text.  Prior to this change, ct_base64_decode() would
return a length of -1 along with allocated storage for an input of
"====".

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3379)