mount_main: Fix empty string check

mount_main: Fix empty string check

gcc 7.3.0 complains:

```
usr/utils/mount_main.c: In function ‘print_mount’:
usr/utils/mount_main.c:46:46: warning: comparison between pointer and zero character constant [-Wpointer-compare]
   if (mnt->mnt_type != NULL && mnt->mnt_type != '\0')
                                              ^~
usr/utils/mount_main.c:46:32: note: did you mean to dereference the pointer?
   if (mnt->mnt_type != NULL && mnt->mnt_type != '\0')
                                ^
usr/utils/mount_main.c:48:46: warning: comparison between pointer and zero character constant [-Wpointer-compare]
   if (mnt->mnt_opts != NULL && mnt->mnt_opts != '\0')
                                              ^~
usr/utils/mount_main.c:48:32: note: did you mean to dereference the pointer?
   if (mnt->mnt_opts != NULL && mnt->mnt_opts != '\0')
                                ^
```

Gbp-Pq: Name mount_main-Fix-empty-string-check.patch