Assorted coverity fixes (#41406)

src/libsystemd/sd-bus/bus-socket.c

@@ -676,6 +676,8 @@ static int bus_socket_read_auth(sd_bus *b) {
                 return -ECONNRESET;
         }
 
+        /* Silence static analyzers, k is bounded by iov size: n - rbuffer_size */
+        assert((size_t) k <= n - b->rbuffer_size);
         b->rbuffer_size += k;
 
         if (handle_cmsg) {
@@ -1453,6 +1455,8 @@ int bus_socket_read_message(sd_bus *bus) {
                 return -EXFULL;
         }
 
+        /* Silence static analyzers, k is bounded by iov size: need - rbuffer_size */
+        assert((size_t) k <= need - bus->rbuffer_size);
         bus->rbuffer_size += k;
 
         if (handle_cmsg) {

src/libsystemd/sd-event/sd-event.c

@@ -3807,6 +3807,9 @@ static int process_signal(sd_event *e, struct signal_data *d, uint32_t events, i
                 if (_unlikely_(!SIGNAL_VALID(si.ssi_signo)))
                         return -EIO;
 
+                /* Silence static analyzers */
+                assert(si.ssi_signo < _NSIG);
+
                 if (e->signal_sources)
                         s = e->signal_sources[si.ssi_signo];
                 if (!s)