ViAppendLine crashes with cursor at end of input on empty line (#797)

Fix #796

Author: springcomp

PSReadLine/ReadLine.vi.cs

@@ -1189,6 +1189,10 @@ private bool IsAtEndOfPhrase()
             {
                 return true;
             }
+            if (_current == _buffer.Length && _buffer[_current - 1] == '\n')
+            {
+                return true;
+            }
             if (_buffer[_current] == '\n')
             {
                 return true;

test/BasicEditingTest.VI.cs

@@ -120,6 +120,19 @@ public void ViDefect628()
                 ));
         }
 
+        [Fact]
+        public void Defect796()
+        {
+            TestSetup(KeyMode.Vi);
+
+            Test("\"\n\n\"", Keys(
+                _.DQuote, _.Enter, _.Escape, CheckThat(() => AssertCursorTopIs(0)),
+                'j', CheckThat(() => AssertCursorTopIs(1)),
+                'o', CheckThat(() => AssertCursorTopIs(2)),
+                _.DQuote
+                ));
+        }
+
         [Fact]
         public void ViChangeMovementUndo()
         {