crypto: fix builtin qcrypto_cipher_free

This was dereferencing a pointer before checking if it was NULL.
Reported-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Reported-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

crypto/cipher-builtin.c

@@ -354,11 +354,13 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
 
 void qcrypto_cipher_free(QCryptoCipher *cipher)
 {
-    QCryptoCipherBuiltin *ctxt = cipher->opaque;
+    QCryptoCipherBuiltin *ctxt;
+
     if (!cipher) {
         return;
     }
 
+    ctxt = cipher->opaque;
     ctxt->free(cipher);
     g_free(cipher);
 }