[midend]修复内存泄漏和Heap-buffer-overflow问题(getexternalfunction中及其隐秘的错误)，修复全局常量标量访问的错误

src/include/midend/IR.h

@@ -923,6 +923,9 @@ public:
     static constexpr uint64_t DefineOpMask = kAlloca | kStore | kPhi;
     return (kind & DefineOpMask) != 0U;
   } 
+  
+  virtual ~Instruction() = default;
+  
   virtual void print(std::ostream& os) const = 0;
 }; // class Instruction
 
@@ -1655,7 +1658,7 @@ class Module {
   }  ///< 获取函数
   Function* getExternalFunction(const std::string &name) const {
     auto result = externalFunctions.find(name);
-    if (result == functions.end()) {
+    if (result == externalFunctions.end()) {
       return nullptr;
     }
     return result->second.get();