hw/virtio/virtio-crypto: Fix op_code assignment in virtio_crypto_create_asym_session

Currently, if the function fails during the key_len check, the op_code
does not have a proper value, causing virtio_crypto_free_create_session_req
not to free the memory correctly, leading to a memory leak.

By setting the op_code before performing any checks, we ensure that
virtio_crypto_free_create_session_req has the correct context to
perform cleanup operations properly, thus preventing memory leaks.

ASAN log:
==3055068==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 512 byte(s) in 1 object(s) allocated from:
    #0 0x5586a75e6ddd in malloc llvm/compiler-rt/lib/asan/asan_malloc_linux.cpp:129:3
    #1 0x7fb6b63b6738 in g_malloc (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x5e738)
    qemu#2 0x5586a864bbde in virtio_crypto_handle_ctrl hw/virtio/virtio-crypto.c:407:19
    qemu#3 0x5586a94fc84c in virtio_queue_notify_vq hw/virtio/virtio.c:2277:9
    qemu#4 0x5586a94fc0a2 in virtio_queue_host_notifier_read hw/virtio/virtio.c:3641:9

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Message-Id: <20240702211835.3064505-1-zheyuma97@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: ZheyuMa

hw/virtio/virtio-crypto.c

@@ -205,6 +205,7 @@ virtio_crypto_create_asym_session(VirtIOCrypto *vcrypto,
     int queue_index;
     uint32_t algo, keytype, keylen;
 
+    sreq->info.op_code = opcode;
     algo = ldl_le_p(&sess_req->para.algo);
     keytype = ldl_le_p(&sess_req->para.keytype);
     keylen = ldl_le_p(&sess_req->para.keylen);
@@ -224,7 +225,6 @@ virtio_crypto_create_asym_session(VirtIOCrypto *vcrypto,
         iov_discard_front(&iov, &out_num, keylen);
     }
 
-    sreq->info.op_code = opcode;
     asym_info = &sreq->info.u.asym_sess_info;
     asym_info->algo = algo;
     asym_info->keytype = keytype;