[ELF] - Do not crash when object has multiple SHT_MIPS_OPTIONS

Issue was revealed by AFl and I was able to generate such object using yaml2obj.

When object has more than one SHT_MIPS_OPTIONS, 
each except the last one is destroyed after placing into Sections array.
Sections array contains dead pointers finally. LLD may crash then.

Differential revision: https://reviews.llvm.org/D25555

llvm-svn: 284227