[DWARF] hoist nullptr checks. NFC

[nickdesaulniers]

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.

[JDevlieghere]

Thanks, LGTM

[dblaikie]

Is there a test case for this codepath? Or should it be an assertion instead?

[probinson]

In D62118#1510389, @dblaikie wrote:

Is there a test case for this codepath? Or should it be an assertion instead?

I think we should keep the check as is, it could be tickled by incorrect use of the API but a unittest should be able to exercise it fairly easily.

llvm/trunk/lib/DebugInfo/DWARF/DWARFFormValue.cpp

case DW_FORM_addr:
     dumpSectionedAddress(AddrOS, DumpOpts, {Value.uval, Value.SectionIndex});
     break;
   case DW_FORM_addrx:
   case DW_FORM_addrx1:
   case DW_FORM_addrx2:
   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>";
     break;
   }
   case DW_FORM_flag_present:
     OS << "true";
     break;