[Sparc] Avoid writing outside array in applyFixup

Summary: If an object file ends with a relocation that is smaller
than 4 bytes we will write outside the Data array and trigger an
"Invalid index" assertion.

Reviewers: jyknight, venkatra

Reviewed By: jyknight

Subscribers: fedor.sergeev, jrtc27, llvm-commits

Differential Revision: https://reviews.llvm.org/D50971

llvm-svn: 340736