Fix error check in GOST engine

Fix error check in GOST engine

The return value of i2d functions can be negative if an error occurs.
Therefore don't assign the return value to an unsigned type and *then*
check if it is negative.

RT#3862

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 90e7cdff3aa66779486914f88333f6601f0c1cf4)