label64 with FULLDEBUG gives problems

When doing WM_LABEL_SIZE=64 and WM_COMPILE_OPTION=Debug there is a problem in reading even the initial controlDict. This traces back to UIPstream::readStringFromBuffer where it tries to read zero bytes from the current pointer (&externalBuf_[externalBufPosition_]). If that pointer points to one-beyond the end of the buffer the call will fail.

Workaround: do not use pointer in case of zero bytes. Note that it only affects the fulldebug checking; the library routine handles it ok.

E.g.

inline Foam::Istream& Foam::UIPstream::readStringFromBuffer(std::string& str) { // Use std::string::assign() to copy content, including '\0'. // Stripping (when desired) is the responsibility of the sending side.

size_t len;
readFromBuffer(len);

if (len == 0)
{
    str.assign(nullptr, len);
}
else
{
    str.assign(&externalBuf_[externalBufPosition_], len);
}