block/qapi: Plug memory leak in dump_qobject() case QTYPE_QERROR

Introduced in commit a8d8ecb.  Spotted by Coverity.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit f25391c)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

block/qapi.c

@@ -471,6 +471,7 @@ static void dump_qobject(fprintf_function func_fprintf, void *f,
         case QTYPE_QERROR: {
             QString *value = qerror_human((QError *)obj);
             func_fprintf(f, "%s", qstring_get_str(value));
+            QDECREF(value);
             break;
         }
         case QTYPE_NONE: