Fix latent bugs in SCHEMATIC and SCH_SEXPR_PLUGIN

Noticed after writing the qa test: when loading a file immediately
after having loaded a previous one, it was resulting in a nullptr
exception.

eeschema/sch_plugins/kicad/sch_sexpr_plugin.cpp

@@ -455,6 +455,8 @@ SCH_SHEET* SCH_SEXPR_PLUGIN::Load( const wxString& aFileName, SCHEMATIC* aSchema
 
     wxASSERT( m_currentPath.size() == 1 );  // only the project path should remain
 
+    m_currentPath.pop(); // Clear the path stack for next call to Load
+
     return sheet;
 }
 

eeschema/schematic.cpp

@@ -60,6 +60,8 @@ void SCHEMATIC::Reset()
 
         project.m_ErcSettings       = nullptr;
         project.m_SchematicSettings = nullptr;
+
+        m_project = nullptr; // clear the project, so we don't do this again when setting a new one
     }
 
     delete m_rootSheet;