lib: sm3: fix coverity error

lib: sm3: fix coverity error

Coverity scan reported:

CID 449815:         Memory - illegal accesses  (OVERRUN)
Overrunning array of 64 bytes at byte offset 64 by dereferencing pointer
"sctx->buffer + partial". [Note: The source code implementation of the
function has been overridden by a builtin model.]

In line: 252
   memset(sctx->buffer + partial, 0, SM3_BLOCK_SIZE - partial);

The respective line should be:

memset(sctx->buffer + partial, 0, SM3_BLOCK_SIZE - partial - 1);

as partial gets incremented by one before.

Signed-off-by: Heiko Schocher <hs@nabladev.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>