extmod/vfs_posix: Fix getcwd() on non-root VFS.

The unwritten API contract expected of a VFS.getcwd() by mp_vfs_getcwd()
is that its return value should be either "" or "/" when the CWD is at
the root of the VFS and otherwise start with a slash and not end with a
slash. This was not correctly implemented in VfsPosix for instances with
a non-empty root - the required leading slash, if any, was cut off
because the root length includes a trailing slash. This would result in
missing slashes in the middle of the return value of os.getcwd() or in
uninitialized garbage from beyond a string's null terminator when the
CWD was at the VFS root.

Signed-off-by: Christian Walther <cwalther@gmx.ch>

extmod/vfs_posix.c

@@ -186,7 +186,14 @@ STATIC mp_obj_t vfs_posix_getcwd(mp_obj_t self_in) {
     if (ret == NULL) {
         mp_raise_OSError(errno);
     }
-    ret += self->root_len;
+    if (self->root_len > 0) {
+        ret += self->root_len - 1;
+        #ifdef _WIN32
+        if (*ret == '\\') {
+            *(char *)ret = '/';
+        }
+        #endif
+    }
     return mp_obj_new_str(ret, strlen(ret));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(vfs_posix_getcwd_obj, vfs_posix_getcwd);