[DWARF] hoist nullptr checks. NFC

Summary:
This was flagged in https://www.viva64.com/en/b/0629/ under "Snippet No.
15" (see under #13). It looks like PVS studio flags nullptr checks where
the ptr is used inbetween creation and checking against nullptr.

Reviewers: JDevlieghere, probinson

Reviewed By: JDevlieghere

Subscribers: RKSimon, hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62118

llvm-svn: 361176