Fix counting error in the set_serial_number weirdo hex handling check
diff --git a/OpenSSL/crypto/x509.c b/OpenSSL/crypto/x509.c
index d5f5946..91550ca 100644
--- a/OpenSSL/crypto/x509.c
+++ b/OpenSSL/crypto/x509.c
@@ -125,7 +125,7 @@
      * is actually the result.  I hope.  -exarkun
      */
     hexstr = PyBytes_AsString(hex);
-    if (hexstr[2] == 'x') {
+    if (hexstr[1] == 'x') {
         /* +2 to skip the "0x" */
         hexstr += 2;
     }