MDEV-15230: column_json breaks cyrillic in 10.1.31

Use unsigned comparison.

mysql-test/r/dyncol.result

@@ -1883,5 +1883,15 @@ SELECT COLUMN_JSON(COLUMN_CREATE('test','First line\nSecond line')) AS json;
 json
 {"test":"First line\u000ASecond line"}
 #
+# MDEV-15230: column_json breaks cyrillic in 10.1.31
+#
+set names utf8;
+create table t1 (b blob);
+insert into t1 values (column_create('description',column_create('title','Описание')));
+select column_json(b) from t1;
+column_json(b)
+{"description":{"title":"Описание"}}
+drop table t1;
+#
 # end of 10.0 tests
 #

mysql-test/t/dyncol.test

@@ -928,6 +928,15 @@ SELECT COLUMN_JSON(COLUMN_CREATE('a',1 AS DECIMAL,'b',1 AS DECIMAL));
 SELECT COLUMN_JSON(COLUMN_CREATE('test','"\\\t\n\Z')) AS json;
 SELECT COLUMN_JSON(COLUMN_CREATE('test','First line\nSecond line')) AS json;
 
+--echo #
+--echo # MDEV-15230: column_json breaks cyrillic in 10.1.31
+--echo #
+set names utf8;
+create table t1 (b blob);
+insert into t1 values (column_create('description',column_create('title','Описание')));
+select column_json(b) from t1;
+drop table t1;
+
 --echo #
 --echo # end of 10.0 tests
 --echo #

mysys/ma_dyncol.c

@@ -3834,7 +3834,7 @@ my_bool dynstr_append_json_quoted(DYNAMIC_STRING *str,
   for (i= 0; i < len; i++)
   {
     register char c= append[i];
-    if (unlikely(c <= 0x1F))
+    if (unlikely(((uchar)c) <= 0x1F))
     {
       if (lim < 5)
         {