Pass a temporary pointer to o2i_SCT_signature from SCT_new_from_base64

Otherwise, |dec| gets moved past the end of the signature by
o2i_SCT_signature and then can't be correctly freed afterwards.
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1548)

crypto/ct/ct_b64.c

@@ -64,6 +64,7 @@ SCT *SCT_new_from_base64(unsigned char version, const char *logid_base64,
 {
     SCT *sct = SCT_new();
     unsigned char *dec = NULL;
+    const unsigned char* p = NULL;
     int declen;
 
     if (sct == NULL) {
@@ -102,7 +103,9 @@ SCT *SCT_new_from_base64(unsigned char version, const char *logid_base64,
         CTerr(CT_F_SCT_NEW_FROM_BASE64, X509_R_BASE64_DECODE_ERROR);
         goto err;
     }
-    if (o2i_SCT_signature(sct, (const unsigned char **)&dec, declen) <= 0)
+
+    p = dec;
+    if (o2i_SCT_signature(sct, &p, declen) <= 0)
         goto err;
     OPENSSL_free(dec);
     dec = NULL;