tool_writeout_json: fix encoding of control characters

Control characters without a special escape sequence e.g. %00 or %06
were being encoded as "u0006" instead of "\u0006".

Ref: https://github.com/curl/trurl/pull/214#discussion_r1257487858
Closes #11414

diff --git a/src/tool_writeout_json.c b/src/tool_writeout_json.c
index ec9c640b5ed73d39acf796149bf4c14ddb1dc438..0e4623fe8694952638555dbd5e368b0417e556ea 100644 (file)
--- a/src/tool_writeout_json.c
+++ b/src/tool_writeout_json.c
@@ -62,7 +62,7 @@ void jsonWriteString(FILE *stream, const char *in, bool lowercase)
       break;
     default:
       if(*i < 32) {
-        fprintf(stream, "u%04x", *i);
+        fprintf(stream, "\\u%04x", *i);
       }
       else {
         char out = *i;