patch 8.2.1191: Vim9: crash when function calls itself

Problem:    Vim9: crash when function calls itself.
Solution:   Add status UF_COMPILING. (closes #6441)

src/structs.h

@@ -1539,6 +1539,7 @@ typedef struct funccall_S funccall_T;
 typedef enum {
     UF_NOT_COMPILED,
     UF_TO_BE_COMPILED,
+    UF_COMPILING,
     UF_COMPILED
 } def_status_T;