fs-util: readlinkat() supports an empty string

From readlinkat(2):
Since Linux 2.6.39, pathname can be an empty string, in which case the
call operates on the symbolic link referred to by dirfd (which should
have been obtained using open(2) with the O_PATH and O_NOFOLLOW flags).

src/basic/fs-util.c

@@ -116,7 +116,11 @@ int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char
 int readlinkat_malloc(int fd, const char *p, char **ret) {
         size_t l = PATH_MAX;
 
-        assert(p);
+        assert(fd >= 0 || fd == AT_FDCWD);
+
+        if (fd < 0 && isempty(p))
+                return -EISDIR; /* In this case, the fd points to the current working directory, and is
+                                 * definitely not a symlink. Let's return earlier. */
 
         for (;;) {
                 _cleanup_free_ char *c = NULL;
@@ -126,7 +130,7 @@ int readlinkat_malloc(int fd, const char *p, char **ret) {
                 if (!c)
                         return -ENOMEM;
 
-                n = readlinkat(fd, p, c, l);
+                n = readlinkat(fd, strempty(p), c, l);
                 if (n < 0)
                         return -errno;