std::bad_alloc in CoinFindDirSeparator() when current working directory doesn't exist

[cyderize]

On Linux (and possibly other platforms), when the current directory does not exist, then getcwd() always fails (and gives errno = ENOENT), meaning that the function enters an infinite loop trying to allocate larger and larger buffers until size overflows into a negative number leading to a std::bad_alloc.

https://github.com/coin-or/CoinUtils/blob/ab8cedf4cf4a163725251666f87276e44f492e81/src/CoinHelperFunctions.hpp#L952-L969

This causes Cbc and many other coin programs to crash on startup since they call CoinFindDirSeparator().

I'm not sure why the directory separator needs to be found in this way (and not just use defines to set it to backslash on Windows and forward slash everywhere else).