Fixes a "buffer overflow detected" abort when compiled with _FORTIFY_SOURCE=1.
The realpath(3) function in glibc checks if the destination buffer is large
enough to hold up to PATH_MAX characters and aborts if that is not the case.
PATH_MAX doesn't have to be defined so assume that it's equal to 4096.
We should really be using pathconf(_PC_PATH_MAX) instead of a hard-coded value
but that means we can no longer use static buffers to hold paths.
