patch 9.0.1218: completion includes functions that don't work

Problem:    Completion includes functions that don't work.
Solution:   Skip functions that are not implemented. (Kota Kato,
            closes #11845)

src/evalfunc.c

@@ -2872,6 +2872,10 @@ get_function_name(expand_T *xp, int idx)
     }
     if (++intidx < (int)ARRAY_LENGTH(global_functions))
     {
+	// Skip if the function doesn't have an implementation (feature not
+	// implemented).
+	if (global_functions[intidx].f_func == NULL)
+	    return (char_u *)"";
 	STRCPY(IObuff, global_functions[intidx].f_name);
 	STRCAT(IObuff, "(");
 	if (global_functions[intidx].f_max_argc == 0)

src/testdir/test_cmdline.vim

@@ -489,6 +489,12 @@ func Test_getcompletion()
   let l = getcompletion('paint', 'function')
   call assert_equal([], l)
 
+  if !has('ruby')
+    " global_functions[] has an entry but it doesn't have an implemention
+    let l = getcompletion('ruby', 'function')
+    call assert_equal([], l)
+  endif
+
   let Flambda = {-> 'hello'}
   let l = getcompletion('', 'function')
   let l = filter(l, {i, v -> v =~ 'lambda'})

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1218,
 /**/
     1217,
 /**/