[DWARF] hoist nullptr checks. NFC
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
Differential Revision: https://reviews.llvm.org/D62118