Fix faulty free

On error, i2o_SCT_signature() and i2o_SCT() free a pointer that may
have wandered off from the start of the allocated block (not currently
true for i2o_SCT_signature(), but has that potential as the code may
change.  To avoid this, save away the start of the allocated block and
free that instead.

Thanks to Guido Vranken for reporting this issue.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2323)

crypto/ct/ct_oct.c

@@ -153,7 +153,7 @@ err:
 int i2o_SCT_signature(const SCT *sct, unsigned char **out)
 {
     size_t len;
-    unsigned char *p = NULL;
+    unsigned char *p = NULL, *pstart = NULL;
 
     if (!SCT_signature_is_complete(sct)) {
         CTerr(CT_F_I2O_SCT_SIGNATURE, CT_R_SCT_INVALID_SIGNATURE);
@@ -177,7 +177,7 @@ int i2o_SCT_signature(const SCT *sct, unsigned char **out)
             p = *out;
             *out += len;
         } else {
-            p = OPENSSL_malloc(len);
+            pstart = p = OPENSSL_malloc(len);
             if (p == NULL) {
                 CTerr(CT_F_I2O_SCT_SIGNATURE, ERR_R_MALLOC_FAILURE);
                 goto err;
@@ -193,14 +193,14 @@ int i2o_SCT_signature(const SCT *sct, unsigned char **out)
 
     return len;
 err:
-    OPENSSL_free(p);
+    OPENSSL_free(pstart);
     return -1;
 }
 
 int i2o_SCT(const SCT *sct, unsigned char **out)
 {
     size_t len;
-    unsigned char *p = NULL;
+    unsigned char *p = NULL, *pstart = NULL;
 
     if (!SCT_is_complete(sct)) {
         CTerr(CT_F_I2O_SCT, CT_R_SCT_NOT_SET);
@@ -224,7 +224,7 @@ int i2o_SCT(const SCT *sct, unsigned char **out)
         p = *out;
         *out += len;
     } else {
-        p = OPENSSL_malloc(len);
+        pstart = p = OPENSSL_malloc(len);
         if (p == NULL) {
             CTerr(CT_F_I2O_SCT, ERR_R_MALLOC_FAILURE);
             goto err;
@@ -250,7 +250,7 @@ int i2o_SCT(const SCT *sct, unsigned char **out)
 
     return len;
 err:
-    OPENSSL_free(p);
+    OPENSSL_free(pstart);
     return -1;
 }