Fix global read overrun error in MBConvTestCase

Use null-terminated copy of the input buffer to create the formatted
message instead of "multiBuffer" itself, which isn't.

tests/mbconv/mbconvtest.cpp

@@ -1147,7 +1147,7 @@ void MBConvTestCase::TestDecoder(
     // make sure the correct output length was calculated
     WX_ASSERT_EQUAL_MESSAGE
     (
-        ("while converting \"%s\"", multiBuffer),
+        ("while converting \"%s\"", inputCopy),
         wideChars,
         outputWritten
     );