Coverity 1508534 & 1508540: misuses of time_t

Avoid problems when the lower 32 bits of time_t roll over by delaying
the cast to integer until after the time delta has been computed.

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #19004)

(cherry picked from commit a6cadcb)

ssl/statem/extensions_clnt.c

@@ -1002,7 +1002,7 @@ EXT_RETURN tls_construct_ctos_psk(SSL *s, WPACKET *pkt, unsigned int context,
                                   X509 *x, size_t chainidx)
 {
 #ifndef OPENSSL_NO_TLS1_3
-    uint32_t now, agesec, agems = 0;
+    uint32_t agesec, agems = 0;
     size_t reshashsize = 0, pskhashsize = 0, binderoffset, msglen;
     unsigned char *resbinder = NULL, *pskbinder = NULL, *msgstart = NULL;
     const EVP_MD *handmd = NULL, *mdres = NULL, *mdpsk = NULL;
@@ -1059,8 +1059,7 @@ EXT_RETURN tls_construct_ctos_psk(SSL *s, WPACKET *pkt, unsigned int context,
          * this in multiple places in the code, so portability shouldn't be an
          * issue.
          */
-        now = (uint32_t)time(NULL);
-        agesec = now - (uint32_t)s->session->time;
+        agesec = (uint32_t)(time(NULL) - s->session->time);
         /*
          * We calculate the age in seconds but the server may work in ms. Due to
          * rounding errors we could overestimate the age by up to 1s. It is

ssl/statem/extensions_srvr.c

@@ -1167,7 +1167,7 @@ int tls_parse_ctos_psk(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
                 s->ext.early_data_ok = 1;
             s->ext.ticket_expected = 1;
         } else {
-            uint32_t ticket_age = 0, now, agesec, agems;
+            uint32_t ticket_age = 0, agesec, agems;
             int ret;
 
             /*
@@ -1209,8 +1209,7 @@ int tls_parse_ctos_psk(SSL *s, PACKET *pkt, unsigned int context, X509 *x,
             }
 
             ticket_age = (uint32_t)ticket_agel;
-            now = (uint32_t)time(NULL);
-            agesec = now - (uint32_t)sess->time;
+            agesec = (uint32_t)(time(NULL) - sess->time);
             agems = agesec * (uint32_t)1000;
             ticket_age -= sess->ext.tick_age_add;