[clang][analyzer] Fix a nullptr dereference when -ftime-trace is used (#139820)

Fixes #139779.

The bug was introduced in #137355 in `SymbolConjured::getStmt`, when
trying to obtain a statement for a CFG initializer without an
initializer. This commit adds a null check before access.

Author: fangyi-zhou

clang/include/clang/StaticAnalyzer/Core/PathSensitive/SymbolManager.h

@@ -103,7 +103,10 @@ class SymbolConjured : public SymbolData {
   const Stmt *getStmt() const {
     switch (Elem->getKind()) {
     case CFGElement::Initializer:
-      return Elem->castAs<CFGInitializer>().getInitializer()->getInit();
+      if (const auto *Init = Elem->castAs<CFGInitializer>().getInitializer()) {
+        return Init->getInit();
+      }
+      return nullptr;
     case CFGElement::ScopeBegin:
       return Elem->castAs<CFGScopeBegin>().getTriggerStmt();
     case CFGElement::ScopeEnd:

clang/test/Analysis/ftime-trace-no-init.cpp

@@ -0,0 +1,5 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,apiModeling %s -ftime-trace=%t.raw.json -verify
+// expected-no-diagnostics
+
+// GitHub issue 139779
+struct {} a; // no-crash