[alpha.webkit.ForwardDeclChecker] Add a missing nullptr check (llvm#171740)

(cherry picked from commit 5f6a5e0)

Author: rniwa

clang/lib/StaticAnalyzer/Checkers/WebKit/ForwardDeclChecker.cpp

@@ -266,10 +266,11 @@ class ForwardDeclChecker : public Checker<check::ASTDecl<TranslationUnitDecl>> {
     while (ArgExpr) {
       ArgExpr = ArgExpr->IgnoreParenCasts();
       if (auto *InnerCE = dyn_cast<CallExpr>(ArgExpr)) {
-        auto *InnerCallee = InnerCE->getDirectCallee();
-        if (isStdOrWTFMove(InnerCallee) && InnerCE->getNumArgs() == 1) {
-          ArgExpr = InnerCE->getArg(0);
-          continue;
+        if (auto *InnerCallee = InnerCE->getDirectCallee()) {
+          if (isStdOrWTFMove(InnerCallee) && InnerCE->getNumArgs() == 1) {
+            ArgExpr = InnerCE->getArg(0);
+            continue;
+          }
         }
       }
       if (auto *UO = dyn_cast<UnaryOperator>(ArgExpr)) {

clang/test/Analysis/Checkers/WebKit/forward-decl-checker.mm

@@ -71,6 +71,10 @@ void rval(Obj&& arg) {
   return objcobj;
 }
 
+void obj_ptr_null_callee(ObjCObj* (*cb)()) {
+  receive_objcobj(cb());
+}
+
 struct WrapperObj {
   Obj* ptr { nullptr };
   // expected-warning@-1{{Member variable 'ptr' uses a forward declared type 'Obj *'}}