uefisign: handle empty sections

uefisign: handle empty sections

loader.efi has an empty set_Xfic section.  Handle it correctly.

```
Sections:
Idx Name          Size      VMA               LMA               File off  Algn
[...]
3 set_Xcom      00000168  00000000000d4000  00000000000d4000  000d0e00  2**2
	      CONTENTS, ALLOC, LOAD, DATA
4 set_Xfic      00000000  00000000000d4168  00000000000d4168  00000000  2**2
	      ALLOC, LOAD, DATA
5 .sdata        00000448  00000000000d5000  00000000000d5000  000d1000  2**2
	      CONTENTS, ALLOC, LOAD, DATA
[...]
```

Reviewed by:	trasz, dab
Reported by:	andy.y.liu@dell.com
Tested by:	andy.y.liu@dell.com
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D29606