patch 8.2.0419: various memory leaks in Vim9 script code

Problem:    Various memory leaks in Vim9 script code.
Solution:   Fix the leaks. (Ozaki Kiichi, closes #5814)
diff --git a/src/vim9.h b/src/vim9.h
index 38286b3..ffa09a7 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -203,7 +203,7 @@
 /*
  * Instruction
  */
-typedef struct {
+struct isn_S {
     isntype_T	isn_type;
     int		isn_lnum;
     union {
@@ -231,7 +231,7 @@
 	loadstore_T	    loadstore;
 	script_T	    script;
     } isn_arg;
-} isn_T;
+};
 
 /*
  * Info about a function defined with :def.  Used in "def_functions".