Added test cases

src/libraries/System.Memory/tests/Base64/Base64DecoderUnitTests.cs

@@ -272,6 +272,23 @@ public void BasicDecodingWithFinalBlockTrueKnownInputInvalid(string inputString,
             Assert.True(Base64TestHelper.VerifyDecodingCorrectness(expectedConsumed, decodedBytes.Length, source, decodedBytes));
         }
 
+        [Theory]
+        [InlineData("�z/T", 0, 0)]                                              // scalar code-path
+        [InlineData("z/Ta123�", 4, 3)]
+        [InlineData("�z/TpH7sqEkerqMweH1uSw==", 0, 0)]                          // Vector128 code-path
+        [InlineData("z/TpH7sqEkerqMweH1uSw�==", 20, 15)]
+        [InlineData("�z/TpH7sqEkerqMweH1uSw1a5ebaAF9xa8B0ze1wet4epo==", 0, 0)]  // Vector256 / AVX code-path
+        [InlineData("z/TpH7sqEkerqMweH1uSw1a5ebaAF9xa8B0ze1wet4epo�==", 44, 33)]
+        public void BasicDecodingNonAsciiInputInvalid(string inputString, int expectedConsumed, int expectedWritten)
+        {
+            Span<byte> source = Encoding.UTF8.GetBytes(inputString);
+            Span<byte> decodedBytes = new byte[Base64.GetMaxDecodedFromUtf8Length(source.Length)];
+
+            Assert.Equal(OperationStatus.InvalidData, Base64.DecodeFromUtf8(source, decodedBytes, out int consumed, out int decodedByteCount));
+            Assert.Equal(expectedConsumed, consumed);
+            Assert.Equal(expectedWritten, decodedByteCount); // expectedWritten == decodedBytes.Length
+        }
+
         [Theory]
         [InlineData("AQID", 3)]
         [InlineData("AQIDBAUG", 6)]