out_gelf: fix crash after missing short_message key

Fix for an illegal free in the gelf output plugin when it handled
a message lacking the short_message key.
In `cb_gelf_flush`: if the input doesn't contain the short_message key
`flb_msgpack_to_gelf` will fail and return NULL. The variable `s` is
not initialized, but still `flb_sds_destroy(s)` is called which
may unexpectedly free some arbitrary pointer.
Fixed by initializing `s`.

Fixes #10086

Author: bpetermannS11

plugins/out_gelf/gelf.c

@@ -312,6 +312,7 @@ static void cb_gelf_flush(struct flb_event_chunk *event_chunk,
         off = log_decoder.offset;
         size = off - prev_off;
         prev_off = off;
+        s = NULL;
 
         map = *log_event.body;