[lldb/DWARF] Fix a crash parsing invalid dwarf (pr49678)

If the debug info is missing the terminating null die, we would crash
when trying to access the nonexisting children/siblings. This was
discovered because the test case for D98619 accidentaly produced such
input.

lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp

@@ -236,6 +236,11 @@ void DWARFUnit::ExtractDIEsRWLocked() {
   }
 
   if (!m_die_array.empty()) {
+    // The last die cannot have children (if it did, it wouldn't be the last one).
+    // This only makes a difference for malformed dwarf that does not have a
+    // terminating null die.
+    m_die_array.back().SetHasChildren(false);
+
     if (m_first_die) {
       // Only needed for the assertion.
       m_first_die.SetHasChildren(m_die_array.front().HasChildren());

lldb/unittests/SymbolFile/DWARF/DWARFUnitTest.cpp

@@ -47,3 +47,40 @@ DWARF:
   ASSERT_NE(die_first, nullptr);
   EXPECT_TRUE(die_first->IsNULL());
 }
+
+TEST(DWARFUnitTest, MissingSentinel) {
+  // Make sure we don't crash if the debug info is missing a null DIE sentinel.
+  const char *yamldata = R"(
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_386
+DWARF:
+  debug_abbrev:
+    - Table:
+        - Code:            0x00000001
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_yes
+          Attributes:
+            - Attribute:       DW_AT_language
+              Form:            DW_FORM_data2
+  debug_info:
+    - Version:         4
+      AddrSize:        8
+      Entries:
+        - AbbrCode:        0x00000001
+          Values:
+            - Value:           0x000000000000000C
+)";
+
+  YAMLModuleTester t(yamldata);
+  ASSERT_TRUE((bool)t.GetDwarfUnit());
+
+  DWARFUnit *unit = t.GetDwarfUnit();
+  const DWARFDebugInfoEntry *die_first = unit->DIE().GetDIE();
+  ASSERT_NE(die_first, nullptr);
+  EXPECT_EQ(die_first->GetFirstChild(), nullptr);
+  EXPECT_EQ(die_first->GetSibling(), nullptr);
+}