ssl_mail_client.c: silence warning, check base64_encode() status
ssl_mail_client.c: silence warning, check base64_encode() status

Found with Clang's `scan-build` tool.

ssl_mail_client.c does a dead store by assigning the return value of
base64_encode() to `len` and not using the value.  This causes
scan-build to issue a warning.

Instead of storing the return value into `len`, store it to `ret`, since
base64_encode() returns a status code, not a length. Also check if the
return value is nonzero and print an error; this silences scan-build.

Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls