Ensure a valid range to string() in PacketReader::getUnquotedText()

Ensure a valid range to string() in PacketReader::getUnquotedText()

In some cases we might have called:

string::string(InputIt first, InputIt last)

with last < first, which is invalid.

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.