[program_options]Iterator decremented past begin

[pgerell]

In format_paragraph the following code appears:

string::const_iterator  line_begin = par.begin();

<snip>

string::const_iterator last_space =
 
find(reverse_iterator<string::const_iterator>(line_end
- 1),
      
reverse_iterator<string::const_iterator>(line_begin - 1),
       ' ')
  .base();

This generates a run-time assertion if compiled with
VC8 and iterators.

[pgerell]

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This was already fixed in CVS