patch 8.2.1391: Vim9: no error for shadowing a script function

Problem:    Vim9: no error for shadowing a script function.
Solution:   Check for already defined items. (closes #6652)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index a6b4a27..25da7f5 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -5462,6 +5462,8 @@
 			semsg(_(e_unknown_var), name);
 			goto theend;
 		    }
+		    else if (check_defined(var_start, varlen, cctx) == FAIL)
+			goto theend;
 		}
 	    }