Fix QA error.

When updating the symbol, we clear the libpins, so remove it to avoid
getting garbage while updating

eeschema/sch_symbol.cpp

@@ -254,6 +254,12 @@ void SCH_SYMBOL::SetLibSymbol( LIB_SYMBOL* aLibSymbol )
     wxCHECK2( !aLibSymbol || aLibSymbol->IsRoot(), aLibSymbol = nullptr );
 
     m_part.reset( aLibSymbol );
+
+    // We've just reset the library symbol, so the lib_pins, which were just
+    // pointers to the old symbol, need to be cleared.
+    for( auto& pin : m_pins )
+        pin->SetLibPin( nullptr );
+
     UpdatePins();
 }
 
@@ -302,7 +308,6 @@ void SCH_SYMBOL::UpdatePins()
         if( !pin->GetAlt().IsEmpty() )
         {
             altPinMap[ pin->GetNumber() ] = pin->GetAlt();
-
             auto altDefIt = pin->GetAlternates().find( pin->GetAlt() );
 
             if( altDefIt != pin->GetAlternates().end() )

qa/tests/eeschema/test_sch_pin.cpp

@@ -197,7 +197,7 @@ BOOST_AUTO_TEST_CASE( AlternatePinRenameUpdates )
     alt.m_Type = ELECTRICAL_PINTYPE::PT_INPUT;
     m_lib_pin->GetAlternates()[ wxS( "ALT1" ) ] = alt;
 
-    m_parent_symbol->UpdatePins();
+    m_parent_symbol->SetLibSymbol( m_parent_part->Flatten().release() );
     m_sch_pin = m_parent_symbol->GetPins()[0];
     m_sch_pin->SetAlt( wxS( "ALT1" ) );
 
@@ -207,7 +207,6 @@ BOOST_AUTO_TEST_CASE( AlternatePinRenameUpdates )
     m_lib_pin->GetAlternates()[ wxS( "ALT1_NEW" ) ] = altNew;
 
     m_parent_symbol->SetLibSymbol( m_parent_part->Flatten().release() );
-    m_parent_symbol->UpdatePins();
 
     SCH_PIN* updatedPin = m_parent_symbol->GetPins()[0];