Fix an assertion in debug build that crashes PSReadLine (#1199)

Author: springcomp

PSReadLine/ViRegister.cs

@@ -60,7 +60,10 @@ public void Record(StringBuilder buffer)
             /// <param name="count"></param>
             public void Record(StringBuilder buffer, int offset, int count)
             {
-                System.Diagnostics.Debug.Assert(offset >= 0 && offset < buffer.Length);
+                System.Diagnostics.Debug.Assert(
+                    offset >= 0 &&
+                    (buffer.Length == 0 || offset < buffer.Length)
+                );
                 System.Diagnostics.Debug.Assert(offset + count <= buffer.Length);
 
                 HasLinewiseText = false;

test/YankPasteTest.VI.cs

@@ -189,6 +189,16 @@ public void ViPasteAfterDeleteWord()
                 ));
         }
 
+        [SkippableFact()]
+        public void ViDeleteLine_EmptyBuffer_Defect1197()
+        {
+            TestSetup(KeyMode.Vi);
+
+            Test("", Keys(
+                _.Escape, "dd", CheckThat(() => AssertLineIs(""))
+            ));
+        }
+
         [SkippableFact]
         public void ViPasteAfterDeleteLine()
         {