Fix error when count exceeds max number of rows in "[count]Y/J" (#4628)

`Position.getDown()` does bounds-checking, so it prevents going past the document's end.

src/actions/commands/actions.ts

@@ -2140,7 +2140,7 @@ export class CommandYankFullLine extends BaseCommand {
   public async exec(position: Position, vimState: VimState): Promise<VimState> {
     const linesDown = (vimState.recordedState.count || 1) - 1;
     const start = position.getLineBegin();
-    const end = new Position(position.line + linesDown, 0).getLineEnd().getLeft();
+    const end = position.getDown(linesDown).getLeft();
 
     vimState.currentRegisterMode = RegisterMode.LineWise;
 
@@ -3075,7 +3075,7 @@ class ActionJoin extends BaseCommand {
       if (position.line + 1 < TextEditor.getLineCount()) {
         startLineNumber = position.line;
         startColumn = 0;
-        endLineNumber = startLineNumber + count;
+        endLineNumber = position.getDown(count).line;
         endColumn = TextEditor.getLineLength(endLineNumber);
       } else {
         startLineNumber = position.line;

test/mode/modeNormal.test.ts

@@ -2016,6 +2016,20 @@ suite('Mode Normal', () => {
     end: ['blah blah', '|blah blah'],
   });
 
+  newTest({
+    title: 'can do [count]Y',
+    start: ['|one', 'two', 'three'],
+    keysPressed: '2Yp',
+    end: ['one', '|one', 'two', 'two', 'three'],
+  });
+
+  newTest({
+    title: 'can do [count]Y if count is larger than EOF',
+    start: ['|one', 'two', 'three'],
+    keysPressed: '100Yp',
+    end: ['one', '|one', 'two', 'three', 'two', 'three'],
+  });
+
   newTest({
     title: 'Can do S',
     start: ['    one', '    tw|o', '    three'],

test/mode/normalModeTests/commands.test.ts

@@ -276,6 +276,20 @@ suite('Mode Normal', () => {
     end: ['one|two'],
   });
 
+  newTest({
+    title: "Can handle 'J' with count",
+    start: ['|one', 'two', 'three', 'four'],
+    keysPressed: '3J',
+    end: ['one two| three', 'four'],
+  });
+
+  newTest({
+    title: "Can handle 'J' with count if count is larger than EOF",
+    start: ['|one', 'two', 'three', 'four'],
+    keysPressed: '100J',
+    end: ['one two three| four'],
+  });
+
   newTest({
     title: "Can handle 'J' in Visual Line mode",
     start: ['on|e', 'two'],