[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

Author: nickdesaulniers

llvm/lib/DebugInfo/DWARF/DWARFFormValue.cpp

@@ -401,12 +401,14 @@ void DWARFFormValue::dump(raw_ostream &OS, DIDumpOptions DumpOpts) const {
   case DW_FORM_addrx3:
   case DW_FORM_addrx4:
   case DW_FORM_GNU_addr_index: {
+    if (U == nullptr) {
+      OS << "<invalid dwarf unit>";
+      break;
+    }
     Optional<object::SectionedAddress> A = U->getAddrOffsetSectionItem(UValue);
     if (!A || DumpOpts.Verbose)
       AddrOS << format("indexed (%8.8x) address = ", (uint32_t)UValue);
-    if (U == nullptr)
-      OS << "<invalid dwarf unit>";
-    else if (A)
+    if (A)
       dumpSectionedAddress(AddrOS, DumpOpts, *A);
     else
       OS << "<no .debug_addr section>";