Bug#29804 UDF parameters don't contain correct string length

 - Fix warnings
 - Export 'check_const_len' from udf_example.dll

sql/udf_example.c

@@ -1115,15 +1115,15 @@ my_bool check_const_len_init(UDF_INIT *initid, UDF_ARGS *args, char *message)
   }
   if (args->args[0] == 0)
   {
-    initid->ptr= "Not constant";
+    initid->ptr= (char*)"Not constant";
   }
   else if(strlen(args->args[0]) == args->lengths[0])
   {
-    initid->ptr= "Correct length";
+    initid->ptr= (char*)"Correct length";
   }
   else
   {
-    initid->ptr= "Wrong length";
+    initid->ptr= (char*)"Wrong length";
   }
   initid->max_length = 100;
   return 0;

sql/udf_example.def

@@ -23,3 +23,4 @@ EXPORTS
   avgcost
   is_const
   is_const_init
+  check_const_len