Fix null-dereference in libedit

Clearing the CurPart sets to null.  Need to keep datasheet checks behind
guards

eeschema/libedit/lib_edit_frame.cpp

@@ -381,8 +381,12 @@ void LIB_EDIT_FRAME::SetCurPart( LIB_PART* aPart )
         delete m_my_part;
         m_my_part = aPart;
 
-        // Datasheet field is special; copy it to the root alias docfilename
-        m_my_part->GetField( DATASHEET )->SetText( aPart->GetRootAlias()->GetDocFileName() );
+        // Datasheet field is special; copy it to the root alias docfilename but watch out
+        // for clearing the aPart
+        if( m_my_part )
+        {
+            m_my_part->GetField( DATASHEET )->SetText( aPart->GetRootAlias()->GetDocFileName() );
+        }
     }
 
     // select the current component in the tree widget