Apply self-imposed path length also to root CAs

Also, some readers of the code find starting the count at 1 for EE
cert confusing (since RFC5280 counts only non-self-issued intermediate
CAs, but we also counted the leaf).  Therefore, never count the EE
cert, and adjust the path length comparison accordinly.  This may
be more clear to the reader.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(cherry picked from commit dc5831da59e9bfad61ba425d886a0b06ac160cd6)

crypto/x509/x509_vfy.c

@@ -517,15 +517,14 @@ static int check_chain_extensions(X509_STORE_CTX *ctx)
         /* check_purpose() makes the callback as needed */
         if (purpose > 0 && !check_purpose(ctx, x, purpose, i, must_be_ca))
             return 0;
-        /* Check pathlen if not self issued */
-        if ((i > 1) && !(x->ex_flags & EXFLAG_SI)
-            && (x->ex_pathlen != -1)
-            && (plen > (x->ex_pathlen + proxy_path_length + 1))) {
+        /* Check pathlen */
+        if ((i > 1) && (x->ex_pathlen != -1)
+            && (plen > (x->ex_pathlen + proxy_path_length))) {
             if (!verify_cb_cert(ctx, x, i, X509_V_ERR_PATH_LENGTH_EXCEEDED))
                 return 0;
         }
         /* Increment path length if not a self issued intermediate CA */
-        if (i == 0 || (x->ex_flags & EXFLAG_SI) == 0)
+        if (i > 0 && (x->ex_flags & EXFLAG_SI) == 0)
             plen++;
         /*
          * If this certificate is a proxy certificate, the next certificate