VirtualFileSystemHook: ensure string is properly null terminated

[jamill]

When the VirtualFileSystem hook receives an error reading data from GVFS
over a named pipe, it will include the pipe message in the error
message. The pipe message is not necassarily null terminated, so we need
to append a null terminating character before passing it to string
formatting functions that expect a null terminated string.

This change makes sure the message is null terminated.

[kewillford]

@benpeart can check my thinking here, but the paths are NUL separated so couldn't this create an invalid path by putting a NUL in the middle of a path? The read from the pipe is not guaranteed to send based on the paths is it?

[jamill]

Valid concern - and maybe we should only add the null character if we are about to write the message out to string.

However, the null character is being added outside of the message, and is not part of the bytesRead (and hence not part of the actual pipe message)?

[benpeart]

The problem here is that GVFS doesn't null terminate the error message passed across the named pipe but the hook assumes it does. We can fix either end. Fixing the hook works.

[benpeart]

This looks better. It's safer to only munge with the buffer in the case of an error.

[jeffhostetler]

That looks fine.