buffer underrun in PDF import routine

Summary:

Whilst porting a portion of the pdf import code over to scribus, I got a segment fault running some of the code. A Close look showed that there was a buffer underrun in one of the functions

Steps to reproduce:

This dpoesn't seem to occur every time in inkscape when I use it, but sometimes when importing pdf files inkscape has crashed, this buffer underrun may be why

What happened?

the application crashed.

What should have happened?

it should have continued and imported the PDF file correctly

Sample attachments:

std::vector::iterator prev_iterator = i - 1 when i starts off as the first iterator begin(), so i-1 is out of bounds.

Version Info:

- Inkscape Version: ... 1.0
- Operating System: ... Windows 
- Operating System version: ...10