[ELF] - Don't crash on multiple SHT_MIPS_REGINFO/SHT_MIPS_ABIFLAGS sections.

In continue of D25555, this patch fixes possible crash when
we have multiple SHT_MIPS_REGINFO or SHT_MIPS_ABIFLAGS sections.
yaml2obj was used to produce such objects.

Differential revision: https://reviews.llvm.org/D25609

llvm-svn: 284376