TLS: Fixed bogus error result

While testing I ran into a situation where a read was returning -1 signaling an
error, but the socket error was 0. This then triggered an assertion failure in
LiteCore.

mbedTLS was returning the code MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY, which our
`translate_mbed_error` method converts to errno 0, i.e. it's not an error at
our level. However, `check_mbed_io` still returned -1.

Fixed by making `check_mbed_io` return 0, not -1, if the error code translates
to 0.

src/mbedtls_context.cpp

@@ -388,7 +388,7 @@ namespace sockpp {
         inline ssize_t check_mbed_io(int mbedResult) {
             if (mbedResult < 0) {
                 clear(translate_mbed_err(mbedResult));     // sets last_error
-                return -1;
+                return last_error() ? -1 : 0;
             }
             return mbedResult;
         }