Don't read uninitialised data for short session IDs.

While it's always safe to read |SSL_MAX_SSL_SESSION_ID_LENGTH| bytes
from an |SSL_SESSION|'s |session_id| array, the hash function would do
so with without considering if all those bytes had been written to.

This change checks |session_id_length| before possibly reading
uninitialised memory. Since the result of the hash function was already
attacker controlled, and since a lookup of a short session ID will
always fail, it doesn't appear that this is anything more than a clean
up.

In particular, |ssl_get_prev_session| uses a stack-allocated placeholder
|SSL_SESSION| as a lookup key, so the |session_id| array may be
uninitialised.

This was originally found with libFuzzer and MSan in
https://boringssl.googlesource.com/boringssl/+/e976e4349d693b4bbb97e1694f45be5a1b22c8c7,
then by Robert Swiecki with honggfuzz and MSan here. Thanks to both.

Reviewed-by: Geoff Thorpe <geoff@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from #2583)

ssl/ssl_lib.c

@@ -2391,13 +2391,21 @@ int SSL_export_keying_material(SSL *s, unsigned char *out, size_t olen,
 
 static unsigned long ssl_session_hash(const SSL_SESSION *a)
 {
+    const unsigned char *session_id = a->session_id;
     unsigned long l;
+    unsigned char tmp_storage[4];
+
+    if (a->session_id_length < sizeof(tmp_storage)) {
+        memset(tmp_storage, 0, sizeof(tmp_storage));
+        memcpy(tmp_storage, a->session_id, a->session_id_length);
+        session_id = tmp_storage;
+    }
 
     l = (unsigned long)
-        ((unsigned int)a->session_id[0]) |
-        ((unsigned int)a->session_id[1] << 8L) |
-        ((unsigned long)a->session_id[2] << 16L) |
-        ((unsigned long)a->session_id[3] << 24L);
+        ((unsigned long)session_id[0]) |
+        ((unsigned long)session_id[1] << 8L) |
+        ((unsigned long)session_id[2] << 16L) |
+        ((unsigned long)session_id[3] << 24L);
     return (l);
 }