Fix that global listener count could be negative under certain circumstances

This fixes a bug that could cause the global listener count of Icecast to
be negative. This was caused due to wrong handling of errors in the
format_prepare_headers function.

Fixes: #2199

Thanks a lot to Simeon Völkel (0xBD4E031CDB4043C9) for reporting
and investigating the bug.

src/format.c

@@ -405,8 +405,12 @@ static int format_prepare_headers (source_t *source, client_t *client)
 
     client->refbuf->len -= remaining;
     if (source->format->create_client_data)
-        if (source->format->create_client_data (source, client) < 0)
+        if (source->format->create_client_data (source, client) < 0) {
+            ICECAST_LOG_ERROR("Client format header generation failed. "
+                "(Likely not enough or wrong source data) Dropping client.");
+            client->respcode = 500;
             return -1;
+        }
     return 0;
 }