Ensure no warnings on release builds.

Change assert(false) into abort().

2020/puzzle-18-01.cc

@@ -52,7 +52,7 @@ private:
       std::cout << "expr_ = " << expr_ << "\n";
       std::cout << "pos_ = " << pos_ << "\n";
       std::cout << "End = " << expr_.substr(pos_) << "\n";
-      assert(false);
+      abort();
     }
   }
 
@@ -86,7 +86,7 @@ private:
       std::cout << "expr_ = " << expr_ << "\n";
       std::cout << "pos_ = " << pos_ << "\n";
       std::cout << "End = " << expr_.substr(pos_) << "\n";
-      assert(false);
+      abort();
     }
   }
 

2020/puzzle-18-02.cc

@@ -62,7 +62,7 @@ private:
       std::cout << "expr_ = " << expr_ << "\n";
       std::cout << "pos_ = " << pos_ << "\n";
       std::cout << "End = " << expr_.substr(pos_) << "\n";
-      assert(false);
+      abort();
     }
   }
 
@@ -96,7 +96,7 @@ private:
       std::cout << "expr_ = " << expr_ << "\n";
       std::cout << "pos_ = " << pos_ << "\n";
       std::cout << "End = " << expr_.substr(pos_) << "\n";
-      assert(false);
+      abort();
     }
   }