Fix a crash when burning things (#36471)

* Make sure material_type::_burn_data is never empty

Perhaps this is a purist in me talking, but a JSON array can be empty but _burn_data must not be empty because material_type::burn_data() returns a reference to one of its elements.

* Fix read past the end of vector in burn_data

#36441

src/material.cpp

@@ -93,7 +93,8 @@ void material_type::load( const JsonObject &jsobj, const std::string & )
         for( JsonObject brn : jsobj.get_array( "burn_data" ) ) {
             _burn_data.emplace_back( load_mat_burn_data( brn ) );
         }
-    } else {
+    } 
+    if( _burn_data.empty() ) {
         // If not specified, supply default
         mat_burn_data mbd;
         if( _fire_resist <= 0 ) {
@@ -255,7 +256,7 @@ bool material_type::reinforces() const
 
 const mat_burn_data &material_type::burn_data( size_t intensity ) const
 {
-    return _burn_data[ std::min<size_t>( intensity, fd_fire.obj().get_max_intensity() ) - 1 ];
+    return _burn_data[ std::min<size_t>( intensity, _burn_data.size() ) - 1 ];
 }
 
 const mat_burn_products &material_type::burn_products() const