Parse errors with zero length give badly-formatted errors

[hildjj]

echo "foo='1' '2'" | peggy -t 1

produces:

Error: Expected "2" but end of input found.
 --> command line:1:2
  |
1 | 1
  |

with no "^" in the third line. When the error is "end of input", the error column will be the same as the length of the line. Add a check for this condition.

I believe GrammarError has the same issue.