Ensure a valid range to string::assign() in PacketReader::xfrBlob()

Ensure a valid range to string::assign() in PacketReader::xfrBlob()

In some cases we could have called:

std::string::assign(InputIterator first, InputIterator last)

with last < first, which is UB:

if the range specified by [first,last) is not valid, it causes undefined behavior

libstdc++ handles that gracefully by throwing an out-of-range exception
but libc++ tries to allocate a negative value of bytes, which in turns
triggers a request for a very large memory allocation, which fails.