[fwutil]: Fix next image mount (sonic-net#1533)

- What I did
Fixed next image mount

- How I did it
Removed lstrip

- How to verify it
root@sonic:/home/admin# fwutil update chassis component <component_name> fw -i next

Signed-off-by: Nazarii Hnydyn <nazariig@nvidia.com>

fwutil/lib.py

@@ -210,7 +210,10 @@ class SquashFs(object):
     OVERLAY_MOUNTPOINT_TEMPLATE = "/tmp/image-{}-overlay"
 
     def __init__(self):
-        image_stem = self.next_image.lstrip(self.OS_PREFIX)
+        image_stem = self.next_image
+
+        if image_stem.startswith(self.OS_PREFIX):
+            image_stem = image_stem[len(self.OS_PREFIX):]
 
         self.fs_path = self.FS_PATH_TEMPLATE.format(image_stem)
         self.fs_rw = self.FS_RW_TEMPLATE.format(image_stem)
@@ -245,6 +248,9 @@ def mount_next_image_fs(self):
         )
         subprocess.check_call(cmd, shell=True)
 
+        if not (os.path.exists(self.fs_rw) and os.path.exists(self.fs_work)):
+            return self.fs_mountpoint
+
         os.mkdir(self.overlay_mountpoint)
         cmd = "mount -n -r -t overlay -o lowerdir={},upperdir={},workdir={} overlay {}".format(
             self.fs_mountpoint,