[NFC] Fix static analyzer concern

Fix static analyzer concern about potential null value
dereference. findBackingIvar() dereferences Prop. PR
checks that Prop exists before calling the function.

Differential Revision: https://reviews.llvm.org/D157429

clang/lib/Analysis/BodyFarm.cpp

@@ -806,7 +806,7 @@ static Stmt *createObjCPropertyGetter(ASTContext &Ctx,
 
   if (!IVar) {
     Prop = MD->findPropertyDecl();
-    IVar = findBackingIvar(Prop);
+    IVar = Prop ? findBackingIvar(Prop) : nullptr;
   }
 
   if (!IVar || !Prop)