Fix uninitialized read in sigalg parsing code

The check for a duplicate value was reading one entry past
where it was supposed to, getting an uninitialized value.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5172)

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index d4c9086..7109741 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -1781,7 +1781,7 @@
 
     /* Reject duplicates */
     for (i = 0; i < sarg->sigalgcnt - 1; i++) {
-        if (sarg->sigalgs[i] == sarg->sigalgs[sarg->sigalgcnt]) {
+        if (sarg->sigalgs[i] == sarg->sigalgs[sarg->sigalgcnt - 1]) {
             sarg->sigalgcnt--;
             return 0;
         }