Fix initialization of SingleCharBuffer and Utf16CharBuffer data

Don't do it at all in the ctor, initializing just the first element of
the array is useless as it's overwritten by EncodeChar() anyhow, so just
leave the task of NUL-terminating the data to this function as well.

It might be even better to just have a ctor taking wxUniChar in these
classes instead and make EncodeChar() a trivial wrapper around it, but
for now just apply the minimal fix to repair the test breakage after the
last commit.

See https://github.com/wxWidgets/wxWidgets/pull/467#issuecomment-310384946

src/common/stringops.cpp

@@ -38,11 +38,13 @@ wxStringOperationsWchar::Utf16CharBuffer wxStringOperationsWchar::EncodeChar(con
     {
         buf.data[0] = (wchar_t)ch.HighSurrogate();
         buf.data[1] = (wchar_t)ch.LowSurrogate();
+        buf.data[2] = L'\0';
     }
     else
     {
         // Assume ch is a BMP character
         buf.data[0] = (wchar_t)ch;
+        buf.data[1] = L'\0';
     }
     return buf;
 }