dump/win_dump: fix use after free of err

It's possible that we'll try to set err twice (or more). It's bad, it
will crash.

Instead, use warn_report().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200324153630.11882-4-vsementsov@virtuozzo.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

dump/win_dump.c

@@ -304,13 +304,11 @@ static void restore_context(WinDumpHeader64 *h,
                             struct saved_context *saved_ctx)
 {
     int i;
-    Error *err = NULL;
 
     for (i = 0; i < h->NumberProcessors; i++) {
         if (cpu_memory_rw_debug(first_cpu, saved_ctx[i].addr,
                 (uint8_t *)&saved_ctx[i].ctx, sizeof(WinContext), 1)) {
-            error_setg(&err, "win-dump: failed to restore CPU #%d context", i);
-            warn_report_err(err);
+            warn_report("win-dump: failed to restore CPU #%d context", i);
         }
     }
 }