Csv Importer - Prevent crash if number of saved columns is higher than actually detected ones.

I ran into this while trying to load a utf-16 encoded file
and loading a saved preset using utf-8. By applying the preset
the columns in the utf-16 file were no longer detected, but
the importer was still trying to read from a preset account column.

gnucash/import-export/csv-imp/gnc-import-tx.cpp

@@ -907,9 +907,13 @@ GncTxImport::accounts ()
             continue;
 
         auto col_strs = std::get<PL_INPUT>(parsed_line);
-        if ((acct_col_it != m_settings.m_column_types.end()) && !col_strs[acct_col].empty())
+        if ((acct_col_it != m_settings.m_column_types.end()) &&
+            (acct_col < col_strs.size()) &&
+            !col_strs[acct_col].empty())
             accts.insert(col_strs[acct_col]);
-        if ((tacct_col_it != m_settings.m_column_types.end()) && !col_strs[tacct_col].empty())
+        if ((tacct_col_it != m_settings.m_column_types.end()) &&
+            (tacct_col < col_strs.size()) &&
+            !col_strs[tacct_col].empty())
             accts.insert(col_strs[tacct_col]);
     }