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

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).

Start in src/CoinHelperFunctions.hpp at CoinFindDirSeparator(), especially the getcwd() buffer-growth loop linked in the issue. Reproduce the failure from a process whose current directory has been removed, then inspect how the existing callers use the result. Done means the function no longer loops until allocation failure or crashes when getcwd() returns ENOENT.

Written by the indexing model from the issue text.