patch 8.2.1945: crash when passing NULL function to reduce()

Problem:    Crash when passing NULL function to reduce().
Solution:   Check for NULL pointer and give an error. (Dominique Pellé,
            closes #7243)

src/list.c

@@ -2552,8 +2552,11 @@ f_reduce(typval_T *argvars, typval_T *rettv)
     }
     else
 	func_name = tv_get_string(&argvars[1]);
-    if (*func_name == NUL)
-	return;		// type error or empty name
+    if (func_name == NULL || *func_name == NUL)
+    {
+	emsg(_(e_missing_function_argument));
+	return;
+    }
 
     vim_memset(&funcexe, 0, sizeof(funcexe));
     funcexe.evaluate = TRUE;