Fix misuse of readlink() buffer causing an uninitialized string ending

This patch should fix #1366
This change affects only Linux.

`readlink()` puts the result in a buffer given as an argument but the null-ending character is not automatically set. This patch add the null character preventing access to uninitialized memory.

Also, even in case of failure of `readlink()` the buffer was returned, an empty string is returned now.

The manpage of `readlink()` says:
> readlink() places the contents of the symbolic link path in the buffer buf, which has size bufsiz. readlink() does not append a null byte to buf. It will truncate the contents (to a length of bufsiz characters), in case the buffer is too small to hold all of the contents.

I do not have a Mac to test but I have the feeling the same kind of errors can occur. Looking at the documentation ([here](https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man3/dyld.3.html)) it says:
> This function returns 0 if the path was successfully copied...

But the actual code is:
```C++
char path[FILENAME_MAX];
uint32_t size = sizeof(path);
if(_NSGetExecutablePath(path, &size) == 0){
	ofLogError() << "buffer too small; need size " <<  size;
}
return path;
```