soter_rand(): cast size_t -> int -> size_t unnecessary and potentially lossy

[veorq]

In https://github.com/cossacklabs/themis/blob/master/src/soter/boringssl/soter_rand.c#L21, soter_rand() takes a size_t and casts it to int to pass it to RAND_bytes(*, size_t); unlike OpenSSL, boringSSL's RAND_bytes() takes a size_t length and not an int. If more than ~4GB are requested then the call will therefore fail to return the right amount.

Likewise, the OpenSSL counterpart may check for overflows in the (needed) cast https://github.com/cossacklabs/themis/blob/master/src/soter/openssl/soter_rand.c.

[vixentael]

thank you, makes sense :)
will fix.