erase functions have bugs

Before calling them would either lead to an exception being thrown
or a null pointer dereference.

Signed-off-by: Eric Curtin <[email protected]>

Author: ericcurtin

include/minja/minja.hpp

@@ -367,11 +367,11 @@ class Value : public std::enable_shared_from_this<Value> {
     }
   }
   void erase(size_t index) {
-    if (array_) throw std::runtime_error("Value is not an array: " + dump());
+    if (!array_) throw std::runtime_error("Value is not an array: " + dump());
     array_->erase(array_->begin() + index);
   }
   void erase(const std::string & key) {
-    if (object_) throw std::runtime_error("Value is not an object: " + dump());
+    if (!object_) throw std::runtime_error("Value is not an object: " + dump());
     object_->erase(key);
   }
   const Value& at(const Value & index) const {