SSL_get_shared_sigalgs: handle negative idx parameter

When idx is negative (as is the case with do_print_sigalgs in
apps/s_cb.c), AddressSanitizer complains about a buffer overflow (read).
Even if the pointer is not dereferenced, this is undefined behavior.

Change the user not to use "-1" as index since the function is
documented to return 0 on out-of-range values.

Tested with `openssl s_server` and `curl -k https://localhost:4433`.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2349)

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index 1534a54..43340d4 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -1684,6 +1684,7 @@
 {
     const SIGALG_LOOKUP *shsigalgs;
     if (s->cert->shared_sigalgs == NULL
+        || idx < 0
         || idx >= (int)s->cert->shared_sigalgslen
         || s->cert->shared_sigalgslen > INT_MAX)
         return 0;