Sync with upstream on i2d_X509_AUX.

Upstream decided to reset *pp on error and to later fix up the other i2d
functions to behave similarly. See upstream's
c5e603ee182b40ede7713c6e229c15a8f3fdb58a.

Change-Id: I01f82b578464060d0f2be5460fe4c1b969124c8e
Reviewed-on: https://boringssl-review.googlesource.com/7844
Reviewed-by: Adam Langley <agl@google.com>

crypto/x509/x_x509.c

diff --git a/crypto/x509/x_x509.c b/crypto/x509/x_x509.c
index f12140f..15af177 100644
--- a/crypto/x509/x_x509.c
+++ b/crypto/x509/x_x509.c
@@ -207,6 +207,7 @@
 int i2d_X509_AUX(X509 *a, unsigned char **pp)
 {
     int length, tmplen;
+    unsigned char *start = pp != NULL ? *pp : NULL;
     length = i2d_X509(a, pp);
     if (length < 0 || a == NULL) {
         return length;
@@ -214,6 +215,8 @@
 
     tmplen = i2d_X509_CERT_AUX(a->aux, pp);
     if (tmplen < 0) {
+        if (start != NULL)
+            *pp = start;
         return tmplen;
     }
     length += tmplen;