json: Report first rather than last parse error

Quiz time!  When a parser reports multiple errors, but the user gets
to see just one, which one is (on average) the least useful one?

Yes, you're right, it's the last one!  You're clearly familiar with
compilers.

Which one does QEMU report?

Right again, the last one!  You're clearly familiar with QEMU.

Reproducer: feeding

    {"abc\xC2ijk": 1}\n

to QMP produces

    {"error": {"class": "GenericError", "desc": "JSON parse error, key is not a string in object"}}

Report the first error instead.  The reproducer now produces

    {"error": {"class": "GenericError", "desc": "JSON parse error, invalid UTF-8 sequence in string"}}

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-24-armbru@redhat.com>

qobject/json-parser.c

@@ -54,13 +54,13 @@ static void GCC_FMT_ATTR(3, 4) parse_error(JSONParserContext *ctxt,
 {
     va_list ap;
     char message[1024];
+
+    if (ctxt->err) {
+        return;
+    }
     va_start(ap, msg);
     vsnprintf(message, sizeof(message), msg, ap);
     va_end(ap);
-    if (ctxt->err) {
-        error_free(ctxt->err);
-        ctxt->err = NULL;
-    }
     error_setg(&ctxt->err, "JSON parse error, %s", message);
 }