BUG/MEDIUM: jws: return size_t in JWS functions

BUG/MEDIUM: jws: return size_t in JWS functions

JWS functions are supposed to return 0 upon error or when nothing was
produced. This was done in order to put easily the return value in
trash->data without having to check the return value.

However functions like a2base64url() or snprintf() could return a
negative value, which would be casted in a unsigned int if this happen.

This patch add checks on the JWS functions to ensure that no negative
value can be returned, and change the prototype from int to size_t.

This is also related to issue #3114.

Must be backported to 3.2.