[clangd] Fix a crash in ExtractFunction tweak.

clang-tools-extra/clangd/refactor/tweaks/ExtractFunction.cpp

const FunctionDecl *findEnclosingFunction(const Node *CommonAnc) {
   for (const Node *CurNode = CommonAnc; CurNode; CurNode = CurNode->Parent) {
     // Don't extract from lambdas
     if (CurNode->ASTNode.get<LambdaExpr>())
       return nullptr;
     if (const FunctionDecl *Func = CurNode->ASTNode.get<FunctionDecl>()) {
       // FIXME: Support extraction from templated functions.
       if (Func->isTemplated())
         return nullptr;
+      if (!Func->getBody())
+        return nullptr;
       for (const auto *S : Func->getBody()->children()) {
         // During apply phase, we perform semantic analysis (e.g. figure out
         // what variables requires hoisting). We cannot perform those when the
         // body has invalid statements, so fail up front.
         if (!S)
           return nullptr;
       }
       return Func;

clang-tools-extra/clangd/unittests/tweaks/ExtractFunctionTests.cpp

 int x = 0;
 }
 F (extracted();)
   )cpp";
   EXPECT_EQ(apply(MacroInput), MacroOutput);
 
   // Shouldn't crash.
   EXPECT_EQ(apply("void f([[int a]]);"), "unavailable");
+  EXPECT_EQ(apply("void f(int a = [[1]]);"), "unavailable");
   // Don't extract if we select the entire function body (CompoundStmt).
   std::string CompoundFailInput = R"cpp(
     void f() [[{
       int a;
     }]]
   )cpp";
   EXPECT_EQ(apply(CompoundFailInput), "unavailable");
 }