pptx: fix custom shape equations import
pptx: fix custom shape equations import

 - guide names can be reused and in that case current one used
   in following equations is the last one

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core