fix add_esp() in the gpt-auto-generator.c (#6251)

fix add_esp() in the gpt-auto-generator.c (#6251)

b9088048b15cd21242b2308498fa865f864bfe45 seems to have broke it

fstab_is_mount_point() returns `true` (1) if the mount point exists and `false` (0) if it doesn't exist.

the change in b9088048 considered that if fstab_is_mount_point() returns 0
the mount point exists.