Initialize dstctx->mgf1_md to NULL in rsa_dupctx function

Signed-off-by: lan1120 <lanming@huawei.com>

Reviewed-by: Todd Short <todd.short@me.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #22795)

providers/implementations/signature/rsa_sig.c

@@ -1006,6 +1006,7 @@ static void *rsa_dupctx(void *vprsactx)
     *dstctx = *srcctx;
     dstctx->rsa = NULL;
     dstctx->md = NULL;
+    dstctx->mgf1_md = NULL;
     dstctx->mdctx = NULL;
     dstctx->tbuf = NULL;
     dstctx->propq = NULL;